Fig. 4.3 Individual- and population-level processes associated with a generic host-microparasite interaction. (a) Processes occurring within individual hosts include: (i) a susceptible stage prior to exposure and infection, (ii) the onset of infection following transmission, and (iii) an increase and peak in parasite particles following replication within the host, which is often associated with a period of illness or disease (shaded box). Once the parasite is detected by the immune system, (iv) host antibodies are produced to help the host recognize and attack parasite stages, and this is followed by (v) a decline in the circulating levels of parasites, following which (vi) host immunity could remain high or gradually decline following recovery. (b) Population-level processes include (i) an initially entirely susceptible host population, (ii) introduction of an infected host, and (iii) an increase and peak in the numbers of infected animals following a classic "epidemic curve." This typical outcome of an SIR model (see Box 4.1) results in declining numbers of susceptible hosts and (iv) an increasing number of recovered and immune hosts.

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