infection: The entry and development of an infectious agent in the body of a person or animal. An infection may be either apparent (called manifest)—the infected person appears to be sick, or inapparent—there is no outward sign that an infectious agent has entered that person at all.
report of a disease: An official report that notifies an appropriate health authority of the occurrence of a disease in a human, or in an animal. Human diseases usually are reported first to the local health authority, such as a county health department.
reservoir: A reservoir of an infectious agent, such as a virus, is any animal, person, plant, soil, substance—or combination of any of these — in which the infectious agent normally lives. In addition, the infectious agent must primarily depend on the reservoir for its survival, and must be able to multiply there. It is from the reservoir that the infectious substance is transmitted to a human or other susceptible host.
risk: A) The chance of being exposed to an infectious agent by its specific transmission mechanism. B) The chance of becoming infected if exposed to an infectious agent by its specific transmission mechanism.
transmission of infectious agents (such as a virus): Any mechanism through which an infectious agent, such as a virus, is spread from a reservoir (or source) to a human being. Usually each type of infectious agent is ordinarily spread by only one or a few of the different mechanisms. There are several types of transmission mechanisms:
Direct transmission: This type of transmission is, at base, immediate. The transfer of the infectious agent is, as the name implies, directly into the body. Different infectious agents may get into the body using different routes. Some routes by which infectious diseases are spread directly include personal contact, such as touching, biting, kissing or sexual intercourse. In these cases the agent enters the body through the skin, mouth, an open cut or sore or sexual organs. Infectious agents may spread by tiny droplets of spray directly into the conjunctiva or the mucus membranes of the eye, nose or mouth during sneezing, coughing, spitting, singing or talking (although usually this type of spread is limited to about within one meter’s distance.) This is called droplet spread.
Indirect transmission: Indirect transmission may happen in any of several ways:
Vehicle-borne transmission: In this situation, a vehicle—that is, an inanimate object or material called in scientific terms a “fomite”, becomes contaminated with the infectious agent. The agent, such as a virus, may or may not have multiplied or developed in or on the vehicle. The vehicle contacts the person’s body. It may be ingested (eaten or drunk), touch the skin, or be introduced internally during surgery or medical treatment. Examples of vehicles that can transmit diseases include cooking or eating utensils, bedding or clothing, toys, surgical or medical instruments (like catheters) or dressings. Water, food, drinks (like milk) and biological products like blood, serum, plasma, tissues or organs can also be vehicles.
Vector-borne transmission: When researchers talk about vectors, often they are talking about insects, which as a group of invertebrate animals carry a host of different infectious agents. (However, a vector can be any living creature that transmits an infectious agent to humans.) Vectors may mechanically spread the infectious agent, such as a virus or parasite. In this scenario the vector—for instance a mosquito— contaminates its feet or proboscis (“nose”) with the infectious agent, or the agent passes through its gastrointestinal tract. The agent is transmitted from the vector when it bites or touches a person. In the case of an insect, the infectious agent may be injected with the insect’s salivary fluid when it bites. Or the insect may regurgitate material or deposit feces on the skin, which then enter a person’s body, typically through a bite wound or skin that has been broken by scratching or rubbing. In the case of some infectious agents, vectors are only capable of transmitting the disease during a certain time period. In these situations, vectors play host to the agent. The agent needs the host to develop and mature or to reproduce (multiply) or both (called cyclopropagative). Once the agent is within the vector animal, an incubation period follows during which the agent grows or reproduces or both, depending on the type of agent. Only after this phase is over does the vector become infective. That is, only then can it transmit an agent that is capable of causing disease in the person.
Airborne transmission: In this type of transmission, infective agents are spread as aerosols, and usually enter a person through the respiratory tract. Aerosols are tiny particles, consisting in part or completely of the infectious agent itself, that become suspended in the air. These particles may remain suspended in the air for long periods of time, and some retain their ability to cause disease, while others degenerate due to the effects of sunlight and dryness. When a person breathes in these particles, they become infected with the agent—especially in the alveoli of the lungs. (see also airborne transmission) Small particles of many different sizes contaminated with the infective agent may rise up from soil, clothes, bedding or floors when these are moved, cleaned or blown by wind. These dust particles may be fungal spores—infective agents themselves—tiny bits of infected feces, or tiny particles of dirt or soil that have been contaminated with the agent. Droplet nuclei can remain in the air for a long time. Droplet nuclei are usually the small residues that appear when fluid emitted from an infected host evaporates. In the case of the virus causing hantavirus pulmonary syndrome, the rodent carriers produce urine. The act of spraying the urine may create the aerosols directly, or the virus particles may rise into the air as the urine evaporates. In other situations, the droplets may occur as an unintended result of mechanical or work processes or atomization by heating, cooling, or venting systems in microbiology laboratories, autopsy rooms, slaughterhouses or elsewhere. Both kinds of particles are very tiny. Larger droplets or objects that may be sprayed or blown but that immediately settle down on something rather than remaining suspended, are not considered to belong to the airborne transmission mechanism. Such sprays are considered direct transmission.
vector: any living creature that transmits an infectious agent to humans.
virus: A virus is an extremely tiny infectious agent that is only able to live inside a cell. Basically, viruses are composed of just two parts. The outer part is a protective shell made of protein. This shell is often surrounded by another protective layer or envelope, made of protein or lipids (fats). The inner part is made of genetic material, either RNA or DNA. A virus does not have any other structures (called organelles) that living cells have, like a nucleus or mitochondria. These organelles are the tiny organs that maintain a cell’s metabolism (life processes). A virus has no metabolism at all. Because a virus lacks organelles, it cannot reproduce by itself. To reproduce, a virus invades a cell within the body of a human or other creature, called the host. Each type of virus has particular types of host creatures and host cells that it will invade successfully. Once within the host cell, the virus uses the cell’s own organelles to produce more viruses. In essence, the virus forces the cell to replicate the virus’ own genetic material and protective shell. Once replicated, the new viruses leave the host cell and are ready to invade others.
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