I have been interested in emerging diseases ever since reading The Hot Zone by Richard Preston, which was so intense that it kept me awake during an entire overnight train ride from Boston to Washington DC. So I was very psyched at Christmas to receive a copy of Spillover by David Quammen. I just finished it, and know a lot more now about how diseases jump from animals to humans.
Quammen uses the book to explore theories about why there seem to be more frequent incidents of humans being infected by animal diseases (think SARS, Ebola, Hendra, Avian Flu, etc.). One of the theories he discusses concerns how increased human development is breaking up large contiguous ecosytems into smaller ecosystems separated by cities, farms, etc.
For example, a large forest might be full of bats that could be carriers of some nasty virus. This forest contains metapopulation of bats, or a series of smaller populations that meet and mingle at their edges. In a metapopulation an infection is likely to be constantly present, but at a low level of incidence. If each smaller population becomes isolated, however, that population will likely go through a boom and bust cycle of infection, with periodic epidemics infecting most members of the population.
If that highly infected population runs into humans, there is increased likelihood of the infection passing to the humans. In other words, if 90% of bats are infected, then there is a higher probability of bats passing their disease to humans than if only 10% of bats are infected.
As development has broken up formerly vast forests into smaller forest segments surrounded by cities and suburbs, we have seen metapopulations of natural disease reservoirs (bats, rats, mice, etc.) broken up into the isolated populations that are more likely to transfer diseases. Hence the increasing number of obscure infections jumping into humans.
What struck me about this theory is the parallel to forest fires. Current wildfire thinking holds that if you put out wildfires, fuel loads will build up and eventually you will get a catastrophic fire that can’t be controlled (like the 2002 Biscuit Fire in Oregon, which burned nearly 500,000 acres; I drove through the edge of that fire, and the smoke turned day into night). But if you let natural fires burn, they will clear out the fuel load and not turn into conflagrations.
So you can have small, more frequent fires, or rare, catastrophic fires. Much like you can have frequent, low levels of infection in your animals, or rare, but massive levels of infection. And in both cases, human intervention in the environment is what moves things from low-level balance to a high-level cyclic system.