Continuous and Discontinuous Epidemics

An epidemic (or a pathosystem) may be either continuous or discontinuous, depending on the availability of host tissue for parasitism. The tissues of an annual species, or the leaves of a deciduous species, suffer discontinuous epidemics. This is because there are regular seasons in which there is no host tissue available to the parasite. If there is a discontinuous pathosystem, the parasite has the problem of surviving that adverse season. It has a second problem of finding new host tissue at the start of a new season. And this second problem is compounded if there is a vertical subsystem, because the parasite must then find a host with a matching vertical resistance.

The parasite does not have this problem with continuous epidemics, in which host tissue is continuously available to the parasite. All the tissues of an evergreen tree, or a rain forest herb, for example, suffer continuous epidemics. With only one matching allo-infection, such a host can remain parasitised, by autoinfection, for the rest of its life. There may be seasonal fluctuations in both the intensity of the parasitism, and the frequency of the auto-infection, but the parasitism never stops completely. Some evergreen trees live for millennia, and they need be successfully allo-infected only once.

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