There are many clinical characteristics of human infections that are relevant to modeling of infection transmission dynamics in populations (Thomas and Weber, 2001) and that determine whether they are endemic (i.e., sustained at similar levels over a long period) or epidemic (i.e., with a substantial excess of cases over and above the norm). The time spent in each stage of infection is particularly important, bearing in mind that the "goal" of an infectious agent is to reproduce and disseminate itself through the population. Thus the length of the period of infectivity and its intensity (generally related to the probability of infecting a contact) are clearly important to establish. However the latent period, the period between being infected and becoming infectious, is also a factor to be considered as this also affects how rapidly an infection may spread in the population. One more factor which may or may not be significant is the length of the prepatient period, the time between infection and symptoms becoming apparent, as the onset of symptoms may result in relative isolation from the rest of the population. Some infections, such as herpes simplex or hepatitis B (HBV), may be symptomatic in some individuals but to a greater or lesser degree asymptomatic in others with corresponding differences shown in infectivity, and the tendency to symptomatic vs. asymptomatic infection may vary according to age, as is shown very strongly in the case of hepatitis B (Medley et al., 2001). HBV is also a good example of an infection that may manifest itself as an initial asymptomatic infection or an acute primary infection (in this case lasting a few weeks) leading to recovery or to a persistent chronic infection which for HBV may last several decades or be effectively lifelong. The existence and duration of immunity following infection, ranging for different infections from several months to more or less lifelong immunity, is a further strong determinant of an infection's transmission dynamics in the population. The case fatality rate may or may not be another important determinant. For example, the extremely rapid virulence of the Ebola virus is probably the reason why it has so far failed to propagate itself beyond its local environment; on the other hand, the substantial mortality arising from HBV infection over the long term does not significantly impede its transmission. All these factors may also vary according to the age of infected individuals, so that the age distribution of infection is one further determinant of transmission dynamics (Anderson and May, 1991).
The route or routes of transmission characteristic of an infection also determine the population dynamics of an infection. The means by which the infection leaves its host, the means by which it is transported from one host to another, and the means by which it enters the new host are topics in themselves, but for the purposes of transmission dynamics modeling routes can be classified as horizontal transmission through direct or very close contact between hosts (e.g., measles, rubella), vertical transmission from mother to child at or around birth (e.g., HBV, toxoplasmosis), sexual transmission between sexual partners (often considered to be in a separate category from horizontal transmission) (e.g., gonorrhea, syphilis), environmental transmission in which the infective agent is disseminated in the air, water, food or soil (e.g., Legionnaire's disease, hepatitis A, hookworm), and vector transmission in which the agent is transported, usually by an arthropod, from one host to another (e.g., malaria, river blindness, dengue fever). Additionally some infections make use of more than one host species to complete their transmission cycle (e.g., snail-human for schistosome flukes). As well as the transmission dynamics themselves, these transmission routes determine the structure of the models to be used to investigate the dynamics; the intensity of these different modes of transmission is also often age-dependent, with such age-dependencies forming another determinant of the epidemiological dynamics (Anderson and May, 1991).
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