Viruses love what we’ve done with the planet

Thanks to us, Ebola has lots of places to live.
Thanks to us, Ebola has lots of places to live.
Image: NIAID
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Washington, DC

Humans like to think of themselves as the dominant force of life on the planet, but stepping into the Outbreak: Epidemics in a Connected World exhibit in the Smithsonian Museum of Natural History, makes you reconsider that perspective. The aim of Outbreaks, which opened in May, isn’t to highlight all the fascinating, bizzare ways the tiniest complete packages of genetic material can make us sick. Instead, it’s designed to explain the perfect storm that we’ve brought upon ourselves: The more of the planet humans take over, the more we inadvertently make it a viral paradise, and a dangerous place for us to live.

For example, the exhibit shows that our vulnerability to viral infection depends on our proximity to other animals and humans. As the global population increases, more of us are moving to ever-growing cities, creating fantastic opportunities for viruses to jump from human host to human host. In cases where the limits of urban areas expand into formerly wild environments, or when forests are cleared for farming, animal contact becomes more common. And when humans and animals share space, they also share microbiology—which is where our trouble starts.

Viruses have a straightforward mission: enter cells, reproduce, burst forth, and repeat. The constant copying eventually destroys so many cells, whatever living thing is acting as the virus’s host will fall ill.

That’d be one thing—bad, but not devastating—if these viruses stayed the same as they perpetuated. Then we could likely, eventually, create vaccines for all of the viruses that infect humans. But, as viruses copy themselves using the machinery of their host’s cells, they constantly mutate. Inevitably, some of these mutations are advantageous, enabling the virus to, for example, spread faster or infect a new type of host. Thanks to mutations, there are over 200 “zoonotic” viruses that can jump between animals and humans, including notorious infections like HIV, Ebola, hepatitis, hanta virus, and several strains of the flu. These are the ones we’re not ready for.

When zoonotic viruses spill over to humans

The more we interact with animals, the more opportunities viruses have to spill over into human populations. Consider the Nipah virus. This month, a Nipah outbreak of 19 cases and 17 deaths in Kerala, India was finally contained. The virus first infected humans in Malaysia in the 1990s, after an economic boom led farmers to try to expand their pig yield. As they chopped down forest to make room for new swine, they encroached on fruit bat territory.

Fruit bats—like all 1,200 member of the bat family—are notoriously good at spreading diseases. In the epidemiology community, they’re known as disease reservoirs, meaning they may appear to be perfectly healthy while carrying all sorts of viruses—like Ebola, rabies, the Hendra virus, and some strains of the flu— that are dangerous to other animals, including humans. Scientists aren’t sure why; it could simply be because bats are prevalent all of the world and have a far-reaching range, or it could be due to something unique about bat biology. One theory suggests that by flying, a bat generates so much heat it essentially trains any virus inhabiting it to tolerate the high temperatures that come with fevers in other hosts, preparing the virus to survive the immune system response of other species.

In any case, fruit bats in Malaysia lived so close to pig farms that they’d eat mangos and leave droppings (of both leftover fruit and excrement) contaminated with Nipah. Pigs picked up the Nipah virus, and passed it along to their human farmers. After an outbreak that caused over 100 deaths, public health officials called for the slaughter of over one million pigs.

Although there haven’t been subsequent outbreaks in Malaysia, they’re still common in places with native fruit bats, like Bangladesh. The most recent India outbreak is suspected to have begun via a bat-infested well (paywall).

Globalization and viral epidemics

Other factors have contributed to viruses’ success. The sudden explosion of global travel due the World War I, for example, played a huge roll in the spread of the 1918 flu. As troops from each side traveled to Europe, there brought strains of a particularly virulent flu with them. In total, that virus killed between 50 and 100 million people over the course of a year, about 5% of the total global population, and even made its way up to the Arctic.

In more recent history, the 2002 SARS-virus outbreak, which had the highest death toll than any other viral epidemic since the 1960s (not including the ongoing HIV/AIDS epidemic) spread rapidly and to various countries thanks to an increasingly globalized economy and society. It moved via airplanes from China to North America, Europe, and Australia, sickening over 8,000 individuals in less than a year.

Additionally, the expanding reach of warmer weather resulting from climate change produces more ideal conditions for both disease vectors, like mosquitos, and the pathogens they transmit.

Humans vs. viruses

Walking through the hall of the exhibit, it’s easy to feel defeated by these microscopic villains. Every panel was an example of how viruses undermined us, and it took time, energy, and usually several hundred deaths before we could figure out how to slow transmission in each outbreak.

As I considered our facility in an increasingly global population, I was grateful for one museum staffer’s words of comfort: All the outbreaks and epidemics featured were the result of poorly-evolved pathogens. The viruses most dangerous to humans end up shooting themselves in their microbial “feet”: They kill their hosts so quickly there isn’t enough time for them to find another. The smartest viruses tend to be the most benign, like strains of herpes simplex, which cause cold sores and herpes. They infect huge swaths of the population, and yet rarely cause medical complications, making them undeserving of any kind of memorialized warning in a museum.

But that’s not to say these long-lived viruses won’t find a way to be both durable and devastating. Meanwhile humans are showing no signs of slowing our population growth any time soon. As we flourish, we have to remember: our success is also a win for the pathogens capable of killing us.