Every year, humans get infected with a few new diseases that could potentially cause a pandemic, and most of these new diseases come from animals. But some types of animals are way more likely to pass along these kinds of diseases than others.
For a potentially pandemic-causing disease to cause a pandemic, two things need to happen: an animal needs to be infected with the disease and then it needs the opportunity to infect a human with it. (The human also needs to actually get infected and spread the infection to others, but that’s another complicated story for another time).
Back to the critters: there are some animals that tend to carry lots of diseases and there are other animals that live in close proximity to humans. But there are also some animals out there that tend to BOTH carry lots of diseases AND be super-likely to come into contact with people. The thing that makes these animals stand out? They live fast and die young.
Compared to animals like humans and whales, which lead long lives and have few babies and invest lots of resources in complex immune systems capable of fighting lots of different diseases, these live fast/die young animals - mostly rodents and small carnivores - don’t live long lives and instead devote their energy towards breeding early and often. As a result, these species tend to have super-simple immune systems with just a few basic tools that mainly combat symptoms like pain and inflammation.
When these animals come down with a nasty bug - like when white-footed mice get infected with the bacteria that causes Lyme Disease - those tools from their basic immune system can fight the symptoms, allowing the animals to go on with their short lives.
But their immune systems don’t actually get rid of the germs themselves, so while the critters' symptoms have been alleviated, they stay infected - and infectious - for a long time, capable of passing along the disease to whomever they come across.
That’s especially bad news because they’re likely to come across … us. Thanks to all that investment in baby-making, live-fast-die-young species multiply quickly. And since lots of babies means lots of genetic diversity, these species can adapt to lots of different habitats, helping them colonize new areas, including those dominated by humans. As a result, they have plenty of opportunities to infect humans with any diseases they’re carrying.
Live-fast-die-young species like raccoons, mice, and brown rats have been responsible for lots of past outbreaks of diseases - and even pandemics like the black death. And as humans continue to disturb natural habitats, scientists expect even more of these outbreaks in the future.
In fact, scientists have recently identified several more animals with those same traits, like the narrow-headed vole and the lowland paca, that live in areas where humans are moving in.
By keeping an eye on the species most likely to pass on a new disease, we can - hopefully - put the brakes on the next outbreak. As we’ve seen over the last few years with COVID-19, preventing the spread of novel diseases is super important. And if you want to understand what would go into a worldwide plan to stop future pandemics from happening, check out Bill Gates’s new book - aptly titled “How To Prevent The Next Pandemic”.