Glossary

A

Acellular vaccine: Listen [MP3]
A vaccine containing partial cellular material as opposed to complete cells.

Acquired Immune Deficiency Syndrome (AIDS): A medical condition where the immune system cannot function properly and protect the body from disease. As a result, the body cannot defend itself against infections (like pneumonia). AIDS is caused by the Human Immunodeficiency Virus (HIV). This virus is spread through direct contact with the blood and body fluids of an infected individual. High risk activities include unprotected sexual intercourse and intravenous drug use (sharing needles). There is no cure for AIDS, however, research efforts are on-going to develop a vaccine.

Active immunity: The production of antibodies against a specific disease by the immune system. Active immunity can be acquired in two ways, either by contracting the disease or through vaccination. Active immunity is usually permanent, meaning an individual is protected from the disease for the duration of their lives.

Acute: Listen [MP3]
A short-term, intense health effect.

Adjuvant: Listen [MP3]
A substance (e.g. aluminum salt) that is added during production to increase the body's immune response to a vaccine.

Adverse events: Undesirable experiences occurring after immunization that may or may not be related to the vaccine.

Advisory Committee on Immunization Practices (ACIP): A panel of 10 experts who make recommendations on the use of vaccines in the United States. The panel is advised on current issues by representatives from the Centers for Disease Control and Prevention, Food and Drug Administration, National Institutes of Health, American Academy of Pediatrics, American Academy of Family Physicians, American Medical Association and others. The recommendations of the ACIP guide immunization practice at the federal, state and local level.

Allergy: A condition in which the body has an exaggerated response to a substance (e.g. food or drug). Also known as hypersensitivity.

Anaphylaxis: Listen [MP3]
An immediate and severe allergic reaction to a substance (e.g. food or drugs). Symptoms of anaphylaxis include breathing difficulties, loss of consciousness and a drop in blood pressure. This condition can be fatal and requires immediate medical attention.

Anthrax: Listen [MP3]
An acute infectious disease caused by the spore-forming bacterium Bacillus anthracis. Anthrax most commonly occurs in hoofed mammals and can also infect humans.

Antibiotic: Listen [MP3]
A substance that fights bacteria.

Antibody: Listen [MP3]
A protein found in the blood that is produced in response to foreign substances (e.g. bacteria or viruses) invading the body. Antibodies protect the body from disease by binding to these organisms and destroying them.

Antigens: Listen [MP3]
Foreign substances (e.g. bacteria or viruses) in the body that are capable of causing disease. The presence of antigens in the body triggers an immune response, usually the production of antibodies.

Antitoxin: Listen [MP3]
Antibodies capable of destroying toxins generated by microorganisms including viruses and bacteria.

Antiviral: Literally "against-virus" -- any medicine capable of destroying or weakening a virus.

Arthralgia: Listen [MP3]
Joint pain.

Arthritis: A medical condition characterized by inflammation of the joints which results in pain and difficulty moving.

Association: The degree to which the occurrence of two variables or events is linked. Association describes a situation where the likelihood of one event occurring depends on the presence of another event or variable. However, an association between two variables does not necessarily imply a cause and effect relationship. The term association and relationship are often used interchangeably. See causal and temporal association.

Asthma: A chronic medical condition where the bronchial tubes (in the lungs) become easily irritated. This leads to constriction of the airways resulting in wheezing, coughing, difficulty breathing and production of thick mucus. The cause of asthma is not yet known but environmental triggers, drugs, food allergies, exercise, infection and stress have all been implicated.

Asymptomatic infection: Listen [MP3]
The presence of an infection without symptoms. Also known as inapparent or subclinical infection.

Attenuated vaccine: Listen [MP3]
A vaccine in which live virus is weakened through chemical or physical processes in order to produce an immune response without causing the severe effects of the disease. Attenuated vaccines currently licensed in the United States include measles, mumps, rubella, polio, yellow fever and varicella. Also known as a live vaccine.

Autism: A chronic developmental disorder usually diagnosed between 18 and 30 months of age. Symptoms include problems with social interaction and communication as well as repetitive interests and activities. At this time, the cause of autism is not known although many experts believe it to be a genetically based disorder that occurs before birth.

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B

B cells: Small white blood cells that help the body defend itself against infection. These cells are produced in bone marrow and develop into plasma cells which produce antibodies. Also known as B lymphocytes.

Bacteria: Tiny one-celled organisms present throughout the environment that require a microscope to be seen. While not all bacteria are harmful, some cause disease. Examples of bacterial disease include diphtheria, pertussis, tetanus, Haemophilus influenzae, and pneumococcal.

Bias: Flaws in the collection, analysis or interpretation of research data that lead to incorrect conclusions.

Biological plausibility: A causal association (or relationship between two factors) is consistent with existing medical knowledge.

Bone marrow: Soft tissue located within bones that produce all blood cells, including the ones that fight infection.

Booster shots: Additional doses of a vaccine needed periodically to "boost" the immune system. For example, the tetanus and diphtheria (Td) vaccine which is recommended for adults every ten years.

Brachial neuritis: Listen [MP3]
Inflammation of nerves in the arm causing muscle weakness and pain.

Breakthrough infection: Development of a disease despite a person's having responded to a vaccine.

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C

Causal association: Listen [MP3]
The presence or absence of a variable (e.g. smoking) is responsible for an increase or decrease in another variable (e.g. cancer). A change in exposure leads to a change in the outcome of interest.

Chickenpox: See Varicella.

Chronic health condition: A health related state that lasts for a long period of time (e.g. cancer, asthma).

Combination vaccine: Two or more vaccines administered in a single dose in order to reduce the number of shots given. For example, the MMR (measles, mumps, rubella) vaccine.

Communicable: That which can be transmitted from one person or animal to another. Also known as infectious.

Crohn's disease: Listen [MP3]
A chronic medical condition characterized by inflammation of the bowel. Symptoms include abdominal pain, diarrhea, fever, loss of appetite and weight loss. The cause of Crohn's disease is not yet known, but genetic, dietary and infectious factors may play a part.

Community immunity: A situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely. Even individuals not vaccinated (such as newborns and those with chronic illnesses) are offered some protection because the disease has little opportunity to spread within the community. Also known as herd immunity.

Conjugate vaccine: Listen [MP3]
The joining together of two compounds (usually a protein and polysaccharide) to increase a vaccine's effectiveness.

Conjunctivitis: Listen [MP3]
Inflammation of the mucous membranes surrounding the eye causing the area to become red and irritated. The membranes may be irritated because of exposure to heat, cold or chemicals. This condition is also caused by viruses, bacteria or allergies.

Contraindication: Listen [MP3]
A condition in a recipient which is likely to result in a life-threatening problem if a vaccine were given.

Convulsion: See Seizure.

Crib or Cot Death: See Sudden Infant Death Syndrome (SIDS).

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D

Deltoid: Listen [MP3]
A muscle in the upper arm where shots are usually given.

Demyelinating disorders: Listen [MP3]
A medical condition where the myelin sheath is damaged. The myelin sheath surrounds nerves and is responsible for the transmission of impulses to the brain. Damage to the myelin sheath results in muscle weakness, poor coordination and possible paralysis. Examples of demyelinating disorders include Multiple Sclerosis (MS), optic neuritis, transverse neuritis and Guillain-Barre Syndrome (GBS).

Diabetes: A chronic health condition where the body is unable to produce insulin and properly breakdown sugar (glucose) in the blood. Symptoms include hunger, thirst, excessive urination, dehydration and weight loss. The treatment of diabetes requires daily insulin injections, proper nutrition and regular exercise. Complications can include heart disease, stroke, neuropathy, poor circulation leading to loss of limbs, hearing impairment, vision problems and death.

Diphtheria: Listen [MP3]
A bacterial disease marked by the formation of a false membrane, especially in the throat, which can cause death.

Disease: Sickness, illness or loss of health.

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E

Efficacy rate: Listen [MP3]
A measure used to describe how good a vaccine is at preventing disease.

Encephalitis: Listen [MP3]
Inflammation of the brain caused by a virus. Encephalitis can result in permanent brain damage or death.

Encephalopathy: Listen [MP3]
A general term describing brain dysfunction. Examples include encephalitis, meningitis, seizures and head trauma.

Epidemic: Listen [MP3]
The occurrence of disease within a specific geographical area or population that is in excess of what is normally expected.

Endemic: Listen [MP3]
The continual, low-level presence of disease in a community

Erythema Multiforme: Listen [MP3]
A medical condition characterized by inflammation of the skin or mucous membranes (including the mouth, throat and eyes). Erthema Multiforme has been reported following infection. Symptoms persist anywhere from 2 days to 4 weeks and include skin lesions, blisters, itching, fatigue, joint pain and fever.

Etiology: Listen [MP3]
The cause of.

Exposure: Contact with infectious agents (bacteria or viruses) in a manner that promotes transmission and increases the likelihood of disease.

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F

Febrile: Listen [MP3]
Relating to fever; feverish.

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G

Guillain-Barre Syndrome (GBS): Listen [MP3]
A rare neurological disease characterized by loss of reflexes and temporary paralysis. Symptoms include weakness, numbness, tingling and increased sensitivity that spreads over the body. Muscle paralysis starts in the feet and legs and moves upwards to the arms and hands. Sometimes paralysis can result in the respiratory muscles causing breathing difficulties. Symptoms usually appear over the course of one day and may continue to progress for 3 or 4 days up to 3 or 4 weeks. Recovery begins within 2-4 weeks after the progression stops. While most patients recover, approximately 15%-20% experience persistent symptoms. GBS is fatal in 5% of cases.

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H

Haemophilus influenzae type b (Hib): Listen [MP3]
A bacterial infection that may result in severe respiratory infections, including pneumonia, and other diseases such as meningitis.

Hepatitis A: A minor viral disease, that usually does not persist in the blood; transmitted through ingestion of contaminated food or water.

Hepatitis B: A viral disease transmitted by infected blood or blood products, or through unprotected sex with someone who is infected.

Hepatitis C: is a liver disease caused by the Hepatitis C virus (HCV), which is found in the blood of persons who have the disease. HCV is spread by contact with the blood of an infected person.

Hepatitis D: is a defective virus that needs the hepatitis B virus to exist. Hepatitis D virus (HDV) is found in the blood of persons infected with the virus.

Hepatitis E: is a virus (HEV) transmitted in much the same way as hepatitis A virus. Hepatitis E, however, does not often occur in the United States.

Herd immunity: See Community immunity.

Herpes Zoster: A disease characterized by painful skin lesions that occur mainly on the trunk (back and stomach) of the body but which can also develop on the face and in the mouth. Complications include headache, vomiting, fever and meningitis. Recovery may take up to 5 weeks. Herpes Zoster is caused by the same virus that is responsible for chickenpox. Most people are exposed to this virus during childhood. After the primary infection (chickenpox), the virus becomes dormant, or inactivated. In some people the virus reactivates years, or even decades, later and causes herpes zoster. Also known as the shingles.

Hives: The eruption of red marks on the skin that are usually accompanied by itching. This condition can be caused by an allergy (e.g. to food or drugs), stress, infection or physical agents (e.g. heat or cold). Also known as uticaria.

Hypersensitivity: Listen [MP3]
A condition in which the body has an exaggerated response to a substance (e.g. food or drug). Also known as an allergy.

Hyposensitivity: Listen [MP3]
A condition in which the body has a weakened or delayed reaction to a substance.

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I

Immune globulin: Listen [MP3]
A protein found in the blood that fights infection. Also known as gamma globulin.

Immune system: The complex system in the body responsible for fighting disease. Its primary function is to identify foreign substances in the body (bacteria, viruses, fungi or parasites) and develop a defense against them. This defense is known as the immune response. It involves production of protein molecules called antibodies to eliminate foreign organisms that invade the body.

Immunity: Protection against a disease. There are two types of immunity, passive and active. Immunity is indicated by the presence of antibodies in the blood and can usually be determined with a laboratory test. See active and passive immunity.

Immunization: Listen [MP3]
The process by which a person or animal becomes protected against a disease. This term is often used interchangeably with vaccination or inoculation.

Immunosupression: Listen [MP3]
When the immune system is unable to protect the body from disease. This condition can be caused by disease (like HIV infection or cancer) or by certain drugs (like those used in chemotherapy). Individuals whose immune systems are compromised should not receive live, attenuated vaccines.

Inactivated vaccine: Listen [MP3]
A vaccine made from viruses and bacteria that have been killed through physical or chemical processes. These killed organisms cannot cause disease.

Inapparent infection: The presence of infection without symptoms. Also known as subclinical or asymptomatic infection.

Incidence: The number of new disease cases reported in a population over a certain period of time.

Incubation period: The time from contact with infectious agents (bacteria or viruses) to onset of disease.

Infectious: Capable of spreading disease. Also known as communicable.

Infectious agents: Organisms capable of spreading disease (e.g. bacteria or viruses).

Inflammation: Redness, swelling, heat and pain resulting from injury to tissue (parts of the body underneath the skin). Also known as swelling.

Inflammatory Bowel Disease (IBD): A general term for any disease characterized by inflammation of the bowel. Examples include colitis and Crohn's disease. Symptoms include abdominal pain, diarrhea, fever, loss of appetite and weight loss.

Influenza: A highly contagious viral infection characterized by sudden onset of fever, severe aches and pains, and inflammation of the mucous membrane.

Intussusception: Listen [MP3]
A type of bowel blockage that happens when one portion of the bowel slides into the next, much like the pieces of a telescope; it is treated in a hospital and may require surgery.

Investigational vaccine: A vaccine that has been approved by the Food and Drug Administration (FDA) for use in clinical trials on humans. However, investigational vaccines are still in the testing and evaluation phase and are not licensed for use in the general public.

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J

Jaundice: Listen [MP3]
Yellowing of the skin and eyes. This condition is often a symptom of hepatitis infection.

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L

Lesion: Listen [MP3]
An abnormal change in the structure of an organ, due to injury or disease.

Live vaccine: A vaccine in which live virus is weakened (attenuated) through chemical or physical processes in order to produce an immune response without causing the severe effects of the disease. Attenuated vaccines currently licensed in the United States include measles, mumps, rubella, shingles (herpes zoster), varicella, and yellow fever. Also known as an attenuated vaccine.

Lupus: A disease characterized by inflammation of the connective tissue (which supports and connects all parts of the body). Chronic swelling of the connective tissue causes damage to the skin, joints, kidneys, nervous system and mucous membranes. The disease begins with fever, joint pain and fatigue. Additional symptoms continue to develop over the years including nausea, fatigue, weight loss, arthritis, headaches and epilepsy. Problems with heart, lung and kidney function may also result. This condition is diagnosed most frequently in young women but also occurs in children.

Lymphocytes: Listen [MP3]
Small white blood cells that help the body defend itself against infection. These cells are produced in bone marrow and develop into plasma cells which produce antibodies. Also known as B cells.

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M

Macrophage: Listen [MP3]
A large cell that helps the body defend itself against disease by surrounding and destroying foreign organisms (viruses or bacteria).

Macular: Listen [MP3]
Skin lesions, normally red-colored.

Measles: A contagious viral disease marked by the eruption of red circular spots on the skin.

Memory Cell: A group of cells that help the body defend itself against disease by remembering prior exposure to specific organisms (e.g. viruses or bacteria). Therefore these cells are able to respond quickly when these organisms repeatedly threaten the body.

Meningitis: Listen [MP3]
Inflammation of the brain and spinal cord that can result in permanent brain damage and death.

Meningoencephalitis: Listen [MP3]
["men in joe en sef uh LIGHT iss"] -- inflammation of the brain and meninges (membranes) that involves the encephalon (area inside the skull) and spinal column.

Microbes: Listen [MP3]
Tiny organisms (including viruses and bacteria) that can only be seen with a microscope.

Mucosal membranes: Listen [MP3]
The soft, wet tissue that lines body openings specifically the mouth, nose, rectum and vagina.

Multiple Sclerosis: Listen [MP3]
Multiple sclerosis (MS) is a disease of the central nervous system characterized by the destruction of the myelin sheath surrounding neurons, resulting in the formation of "plaques." MS is a progressive and usually fluctuating disease with exacerbations (patients feeling worse) and remissions (patients feeling better) over many decades. Eventually, in most patients, remissions do not reach baseline levels and permanent disability and sometimes death occurs. The cause of MS is unknown. The most widely held hypothesis is that MS occurs in patients with a genetic susceptibility and that some environmental factors "trigger" exacerbations. MS is 3 times more common in women than men, with diagnosis usually made as young adults. Also see demyelinating disorders.

Mumps: Acute contagious viral illness marked by swelling, especially of the parotid glands.

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N

Neuritis: Listen [MP3]
Inflammation of the nerves.

Neuropathy: Listen [MP3]
A general term for any dysfunction in the peripheral nervous system. Symptoms include pain, muscle weakness, numbness, loss of coordination and paralysis. This condition may result in permanent disability.

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O

Optic neuritis: Listen [MP3]
A medical condition where vision deteriorates rapidly over hours or days. One or both eyes may be affected. This condition results for the demyelination of optic nerves. In most cases, the cause of optic neuritis is unknown. Patients may regain their vision or be left with permanent impairment. Also see demyelinating disorders.

Orchitis: Listen [MP3]
A complication of mumps infection occurring in males (who are beyond puberty). Symptoms begin 7-10 days after onset of mumps and include inflammation of the testicles, headache, nausea, vomiting, pain and fever. Most patients recover but in rare cases sterility occurs.

Otitis Media: Listen [MP3]
A viral or bacterial infection that leads to inflammation of the middle ear. This condition usually occurs along with an upper respiratory infection. Symptoms include earache, high fever, nausea, vomiting and diarrhea. In addition, hearing loss, facial paralysis and meningitis may result.

Outbreak: Sudden appearance of a disease in a specific geographic area (e.g. neighborhood or community) or population (e.g., adolescents).

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P

Pandemic: An epidemic occurring over a very large geographic area.

Papular: Listen [MP3]
Marked by small red-colored elevation of the skin.

Passive immunity: Protection against disease through antibodies produced by another human being or animal. Passive immunity is effective, but protection is generally limited and diminishes over time (usually a few weeks or months). For example, maternal antibodies are passed to the infant prior to birth. These antibodies temporarily protect the baby for the first 4-6 months of life.

Pathogens: Organisms (e.g. bacteria, viruses, parasites and fungi) that cause disease in human beings.

Pertussis: Listen [MP3]
(whooping cough) Bacterial infectious disease marked by a convulsive spasmodic cough, sometimes followed by a crowing intake of breath.

Petechiae: Listen [MP3]
["pe TEEK ee ay"] -- a tiny reddish or purplish spot on the skin or mucous membrane, commonly part of infectious diseases such as typhoid fever.

Placebo: A substance or treatment that has no effect on human beings.

Pneumonia: Listen [MP3]
Inflammation of the lungs characterized by fever, chills, muscle stiffness, chest pain, cough, shortness of breath, rapid heart rate and difficulty breathing.

Poliomyelitis: Listen [MP3]
(polio) An acute infectious viral disease characterized by fever, paralysis, and atrophy of skeletal muscles.

Polysaccharide vaccines: Listen [MP3]
Vaccines that are composed of long chains of sugar molecules that resemble the surface of certain types of bacteria. Polysaccharide vaccines are available for pneumococcal disease, meningococcal disease and Haemophilus Influenzae type b.

Potency: A measure of strength.

Precaution: A condition in a recipient which may result in a life-threatening problem if the vaccine is given, or a condition which could compromise the ability of the vaccine to produce immunity.

Prevalence: The number of disease cases (new and existing) within a population over a given time period.

Prodromal: Listen [MP3]
An early symptom indicating the onset of an attack or a disease.

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Q

Quarantine: The isolation of a person or animal who has a disease (or is suspected of having a disease) in order to prevent further spread of the disease.

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R

Recombinant: Listen [MP3]
Of or resulting from new combinations of genetic material or cells; the genetic material produced when segments of DNA from different sources are joined to produce recombinant DNA.

Reye Syndrome: Listen [MP3]
Encephalopathy (general brain disorder) in children following an acute illness such as influenza or chickenpox. Symptoms include vomiting, agitation and lethargy. This condition may result in coma or death.

Residual Seizure Disorder (RSD): See seizures.

Risk: The likelihood that an individual will experience a certain event.

Rotavirus: Listen [MP3]
A group of viruses that cause diarrhea in children.

Rubella: (German measles) Viral infection that is milder than normal measles but as damaging to the fetus when it occurs early in pregnancy.

Rubeola: Listen [MP3]
See Measles.

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S

Seroconversion: Listen [MP3]
Development of antibodies in the blood of an individual who previously did not have detectable antibodies.

Serology: Listen [MP3]
Measurement of antibodies, and other immunological properties, in the blood serum.

Serosurvey: Listen [MP3]
Study measuring a person's risk of developing a particular disease.

Seizure: The sudden onset of a jerking or staring spell. Many seizures following a vaccination are caused by fever. Seizures are also known as convulsions.

Severe Combined immune Deficiency (SCID): Included in a group of rare, life-threatening disorders caused by at least 15 different single gene defects that result in profound deficiencies in T- and B- lymphocyte function.

Shingles: See herpes zoster.

Side Effect: Undesirable reaction resulting from immunization.

Smallpox: An acute, highly infectious, often fatal disease caused by a poxvirus and characterized by high fever and aches with subsequent widespread eruption of pimples that blister, produce pus, and form pockmarks. Also called variola.

Strain: A specific version of an organism. Many diseases, including HIV/AIDS and hepatitis, have multiple strains.

Subclinical infection: The presence of infection without symptoms. Also known as inapparent or asymptomatic infection.

Sudden Infant Death Syndrome (SIDS): The sudden and unexpected death of a healthy infant under 1 year of age. A diagnosis of SIDS is made when an autopsy cannot determine another cause of death. The cause of SIDS is unknown. Also known as "crib" or "cot" death.

Susceptible: Unprotected against disease.

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T

Temporal association: Two or more events that occur around the same time but may be unrelated, chance occurrences.

Teratogenic: Listen [MP3]
Of, relating to, or causing developmental malformations.

Tetanus: Listen [MP3]
Toxin-producing bacterial disease marked by painful muscle spasms.

Thimerosal: Listen [MP3]
Thimerosal is a mercury-containing preservative used in some vaccines and other products since the 1930's. There is no convincing evidence of harm caused by the low concentrations of thimerosal in vaccines, except for minor reactions like redness and swelling at the injection site. However, in July 1999, the Public Health Service agencies, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure. Today, all routinely recommended childhood vaccines manufactured for the U.S. market contain either no thimerosal or only trace amounts with the exception of some flu vaccines. There are thimerosal-free influenza vaccines available.

Typhoid Fever: Typhoid fever is a life-threatening illness caused by the bacterium Salmonella Typhi. Persons with typhoid fever carry the bacteria in their bloodstream and intestinal tract.

Titer: Listen [MP3]
The detection of antibodies in blood through a laboratory test.

Transverse Myelitis: Listen [MP3]
The sudden onset of spinal cord disease. Symptoms include general back pain followed by weakness in the feet and legs that moves upward. There is no cure and many patients are left with permanent disabilities or paralysis. Transverse Myelitis is a demyelinating disorder that may be associated with Multiple Sclerosis (MS). Also see demyelinating disorders.

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U

Urticaria: Listen [MP3]
The eruption of red marks on the skin that are usually accompanied by itching. This condition can be caused by an allergy (e.g. to food or drugs), stress, infection or physical agents (e.g. heat or cold). Also known as hives.

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V

Vaccination: Listen [MP3]
Injection of a killed or weakened infectious organism in order to prevent the disease.

Vaccinia: Listen [MP3]
A virus related to the smallpox and cowpox viruses, which is used in smallpox vaccine.

Vaccine: Listen [MP3]
A product that produces immunity therefore protecting the body from the disease. Vaccines are administered through needle injections, by mouth and by aerosol.

Vaccine Adverse Event Reporting System (VAERS): A database managed by the Centers for Disease Control and Prevention and the Food and Drug Administration. VAERS provides a mechanism for the collection and analysis of adverse events associated with vaccines currently licensed in the United States. Reports to VAERS can be made by the vaccine manufacturer, recipient, their parent/guardian or health care provider. For more information on VAERS call (800) 822-7967.

Vaccine Safety Datalink Project (VSD): In order to increase knowledge about vaccine adverse events, the Centers for Disease Control and Prevention have formed partnerships with eight large Health Management Organizations (HMOs) to continually evaluate vaccine safety. The project contains data on more than 6 million people. Medical records are monitored for potential adverse events following immunization. The VSD project allows for planned vaccine safety studies as well as timely investigations of hypothesis.

Varicella: Listen [MP3]
(Chickenpox) -- An acute contagious disease characterized by papular and vesicular lesions.

Variola: Listen [MP3]
See smallpox.

Vesicular: Listen [MP3]
Characterized by small elevations of the skin containing fluid (blisters).

Viremia: Listen [MP3]
The presence of a virus in the blood.

Virulence: Listen [MP3]
The relative capacity of a pathogen to overcome body defenses.

Virus: A tiny organism that multiplies within cells and causes disease such as chickenpox, measles, mumps, rubella, pertussis and hepatitis. Viruses are not affected by antibiotics, the drugs used to kill bacteria.

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W

Waning Immunity: The loss of protective antibodies over time.

Whooping Cough: See Pertussis.

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