The simple answer to the question posed in the title is almost always “animals”. It’s a bit of a cop-out, though. If we really want to know where previously unrecognized infections come from, and how they jump into humans and stay circulating within our population, we have to dig a bit deeper. In my view, the real answer involves a category of proteins called adhesins. These proteins allow infectious microbes to “stick” to our tissues at the cellular level. If our cell surfaces are the fuzzy band of a strip of Velcro, adhesin proteins would be the corresponding hooked band. When they come together, they attach specifically to each other, just as Velcro bands do.
The process of viral attachment, complete with Velcro addendum
What do adhesins and Velcro have to do with new infectious diseases? It comes down to specificity of stickiness, so to speak. In order to infect a cell, viral pathogens in particular invariably must stick to host cells. It turns out that all cells in the animal kingdom do not have all the same size, shape, and texture of Velcro bands. Only a band that matches the corresponding hooked band on the viral adhesin will allow successful infection. By and large, the cellular Velcro bands on human cells do not overlap much with the viral Velcro bands from animal viruses; therefore, most animal viruses do not infect humans, and vice versa. Our Velcro bands simply don’t match up…except when they do.
This is where Middle Eastern Respiratory Syndrome (MERS) coronavirus, Severe Acute Respiratory Syndrome (SARS) coronavirus, and bats come into the story. Both MERS and SARS coronaviruses have recently emerged in the human populations and cause severe viral pneumonia with death rates upward of 30%. Both viruses naturally infect bats, but have adhesins that are quite generalist. In other words, their Velcro bands are able to adhere with less specificity than most other viruses. They bind to bat cells and to human cells, and thus these viruses were able to make the jump from bats to humans (note: in both cases an additional animal served as an intermediary; civet cats for SARS, and camels for MERS). SARS emerged only once, and has not yet reemerged. MERS, however, jumped into humans in 2012 and still circulates in the population to this day. It is largely confined to the Middle East, but cases have occurred in other parts of the world including the United States, Europe, and South Korea. A new study indicates that there are additional Coronaviruses in bat populations with adhesins that are capable of binding humans cells. In other words, the next MERS or SARS already exists. It is waiting, biding its time in its natural host (the horseshoe bat) for the opportune moment.
The Chinese horseshoe bat [Image: S.C. Bisserot, F.R.P.S.]