Why so many diseases?
... [C]an it be seriously suggested that a rather uniform area of Amazonian rain forest provides, in 3.5 hectares of land, anything like 179 separate ecological niches for trees? ... Pest pressure is the inevitable, ubiquitous factor in evolution which makes for an apparently pointless multiplicity of species in all areas in which it has time to operate.
J. B. Gillett, 'Pest pressure, an underestimated
factor in evolution', 1962.
Why is our species afflicted by so many diseases? And why do these diseases swarm with such diversity in the tropics and not in the temperate zones? There are trivial answers to these questions, of course. The tropics are hot and humid, which allows disease organisms to multiply swiftly, so that they can easily contaminate food and water supplies. Because of the plentiful moisture everywhere, the fragile parasites are less likely to die from desiccation when they are exposed outside the body of their host. Further, they can be passed easily from one host to another on films of water or sweat. Seasons are not pronounced, so the parasites are not killed off by cold weather. And because of the sheer abundance of life there are far more possible alternative vectors for disease, ranging from mosquitoes and biting flies through ticks to freshwater snails. As a consequence, diseases in the tropics do not need to be as sophisticated as those in temperate zones.
Because it is not as demanding to be a tropical disease, the tropics provide plentiful opportunities for disease organisms that would have little chance in the temperate zones. We have already seen many