So, for example, when farming was first developed thousands of years ago, we lived near animals for the first time. That allowed us to acquire the antigens.4 Further on in our history, when people started to congregate in cities, disease could spread rapidly for the first time. And in more recent years, things like global air travel, deforestation, wars, changes in agricultural practices, and so on, have changed our relationship with flora and fauna. So new diseases emerged. HIV is a great example. It looks very likely that the change in land use in West Africa—when logging in proximity to monkey habitats—people acquired their viruses.
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Why do some pathogens infect millions while others do not?
Disease is related to population size and density.
Holmes: There are a number of very critical evolutionary parameters. One of the most important—and why I mentioned previously that human evolutionary ecology has changed—is population size and density. From a human ecology standpoint, the more hosts you have, and the denser they are, the more likely the pathogen can get through and spread.
So put in another way, when you are infected by a pathogen, the virus needs a new host to infect to keep itself going. If you have a small population, that is not that likely to happen. If you have a bigger population, you have more chances of transmitting the disease.
Measles can’t survive in some locations.
There have been some amazing studies of measles in populations. Some years ago scientists looked at islands and discovered an amazing statistic. They showed that measles can sustain itself on islands with a population size of something like 300,000 people.
Below that number, measles dies out because there are not enough hosts for the virus to maintain itself. Above 300,000, it can keep itself going. So ecology plays a big role in disease.
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