Animal-to-human transmission: how diseases like mpox make people sick

In 2022, on the heels of the global COVID-19 pandemic, came another highly contagious disease sending people running for the hand sanitizer. Monkeypox—now renamed mpox by the World Health Organization (WHO)¹—is an ominous sounding disease with itchy blisters that look even worse. But does it really come from monkeys? And how does it infect humans?

Turns out animal-to-human transmission is pretty common. People catch a number of common illnesses this way—including diseases like like rabies, Ebola, hantavirus, and many others. But some have origins that might surprise you.

Let’s use the flu as an example. Scientists believe influenza originally came from a combination of pigs and bird cell viruses4 (remember the avian flu outbreak in 2008 and the swine flu outbreak in 2009?). Today, of course, it’s heavily spread through human contact and respiratory droplets, but new flu strains can originate from animals.⁴

Another common disease we’re all intimately familiar with now, COVID-19 (or rather the coronavirus that causes it), is thought to have its origin in bats.⁵

Toxoplasmosis, which is actually a parasite, can be contracted by humans through cat feces.⁶ Hence the warning for pregnant women to let someone else clean the litter box.

Let’s get back to mpox. Interesting fact: it’s believed that the first worldwide human mpox case occurred in Central Africa in the 1970s.⁷ However, the virus was first discovered 2 decades earlier when monkeys in a research laboratory in Denmark were infected during an outbreak. That case inspired its name, but the natural reservoirs where this virus lives and reproduces are actually African rodents—monkeys are likely only accidental hosts.

The first case of mpox in the United States was in 2003 in Wisconsin and stemmed from a new pet prairie dog that was unknowingly housed with infected Gambian rats imported from Africa.⁸ And while the original case came from animal hosts, the disease now spreads through human-to-human contact. This fascinating history led to the 2022 name change from monkeypox to mpox, which helps cut the connection to monkeys when talking about the disease.

Between COVID-19’s bat origins and mpox’s prairie dogs and rats, it seems like we’re hearing about animal-to-human transmission more and more frequently. Two important factors enabling such transmission are climate change and deforestation.⁹ We often think of these two activities as raising global temperatures and causing sea levels to rise, but they’re also responsible for displacing animals from their homes, which can play a big role in spreading animal-to-human diseases.

Because of climate change and deforestation, animals and insects are being forced out of their habitats and into places more populated by humans.⁹ These animals and insects carry diseases less common to humans. Humans are also driving deeper into previously uninhabited areas. Now that we’re living in closer proximity with animals, there’s a higher chance for humans to be infected for the first time while new viruses and parasites are mutating to unlock and invade our cells.

The question should be, can we stop them? Most likely not. Given the momentum of climate change, and given that people can travel halfway across the world in a day, global outbreaks of new diseases present a real threat to society.

That’s not to say mpox will become a pandemic, or that animal-to-human diseases will run amok. But global initiatives to slow climate change, reduce deforestation, and develop new vaccines could also help slow the spread of animal-borne disease.