Recognition of infectious diseases is a part of epidemology, discussed in the previous section. Therefore it again relies on a large amount of statistical data. Usually in this case the statistical data is collected from doctors and the like who acquire it from visiting patients. The patients may have signs, symptoms, or both. Signs are objective and directly observable changes in an individual. A fever, something that can be quantitativly measured with a thermometer and compared to a norm is an example. Symptoms are subjective changes experienced and reported by a patient. This could be stomach pains, constipation, and so on. The signs and symptoms combined are a syndrom. By analysis of these signs and symptoms doctors have a first clue into the nature of an illness.

In addition to this, evaluation is aided by the fact that every disease follows a characteristic sequence of events. There is an incubation period in which the infection has occured but no signs or symptoms are apparent. Time is highly variable in this phase. Following this is the prodromal stage, where the signs and the symptoms begin to appear but they are not clearly identifiable. This is generally when the affected individual is the most contagious and when the general host defenses are fully operating. Following this phase is the illness phase, in which the disease is the most severe with clear signs and symptoms. By this time there is an active immune response, which hopefully will be enough to lead to the convalescene phase, where the disease is declining. By looking at the signs, symptoms, time length of each phase, and various other data medical practioners can begin to deduce the cause of an illness.

If this illness is not treated, an epidemic may result. There are two major types of epidemics, common-source epidemics and prpagated epidemics. Common source epidemics are generally not transmitted but instead result from a single contaminated source of food or water. This is usually an example of an outbreak and features a rapid increase of the disease with a rapid decline. A propagated epidemic starts from one person and goes through the population. The incidence of the disease declines as the number of susceptible people decline. Because of this, this type of epidemic can not occur if most of the population is not initially susceptible. This is called herd immunity, and investigation of this indicates that no propagated epidemic will occur if seventy percent (the threshold density) of the population is resistant. This is because there are not enough good carriers available for the pathogens to get spread. Unfortunately (for us) pathogens can circumvent this problem by changing their antigens. A major change is an antigenic shift, while a minor one is antigenic drift. By changing antigens a pathogen may then be (at worst) unrecognized by the host immune system and will (at least) not be subject to immediate attack.

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