Here’s where scientists think the next epidemic is most likely to start

Viruses jumping from animals to humans have been the starting point of numerous outbreaks, from Ebola to Zika. Given the similarity of SARS-CoV-2 to coronaviruses found in bats, this probably marked the beginning of COVID-19 too.

We know that viruses have passed from animals to humans throughout history, and will continue to do so. But the factors that influence the geographical origin of these events is less clear, despite being highly important. Knowing where they occur can help us understand the factors behind a virus crossing species, in particular, by looking at the traits of viruses circulating in the ecosystem where the jump happened.

But identifying a virus’s origin is sometimes difficult. Human movement is constant and wide-ranging, which means that the first case of a disease can be hundreds, if not thousands of miles away from where transmission into humans started. Given this, where should we be looking for the virus that might cause the next epidemic?

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Beyond Africa and Asia

Generally, viruses emerge where humans and animals that carry viruses intersect. Repeated interaction between people, these animals or insects, and the wider environment in which the virus circulates increases the opportunity for a jump across species. These jumps are believed to be rare, and probably happen due to a specific set of circumstances that cannot necessarily be predicted.

Humans are exposed to viruses all the time. Most of these exposures lead to a “dead-end infection,” where the virus isn’t passed on. Occasionally, though, the virus may be able to replicate and be transmitted to a new host, or if vector-borne, to an insect that establishes a novel and functional transmission cycle.

This happens all over the world, though recent headline-grabbing outbreaks give the impression that viruses emerge in some places more than others. In particular, the seriousness of outbreaks such as Sars in Asia and Ebola in Africa makes it look like these are the only places where it happens. This isn’t the case.

For example, the Schmallenberg virus, which primarily infects livestock and causes spontaneous abortion in infected animals, recently appeared in Europe. And while we don’t hear much about viruses emerging from South America, it does happen. The Venezuelan equine encephalitis virus and the Mayaro virus have repeatedly caused outbreaks in South and Central America. It’s only because these diseases haven’t spread beyond the Americas that they aren’t more widely known.

A further factor that has prevented the Mayaro virus from gaining more attention is that it has very similar symptoms to disease caused by another virus – chikungunya. It’s also often misdiagnosed as dengue fever, meaning the true number of Mayaro cases isn’t being reported.

An Aedes aegypti mosquito feeding on a human.