Zoonotic Diseases

Background 

The source of some of the world’s most devastating and deadly illnesses, zoonotic diseases are infections that spread between animals and people. The diseases may be caused by a variety of pathogens—viruses, bacteria, fungi, or parasites—and can originate in creatures ranging from ticks and mosquitoes to pigs and bats.  

Because animals provide a huge reservoir in which diseases can mutate and evolve, many emerging diseases have never been seen before, meaning they are more likely to be dangerous to humans and less likely to have existing treatments. 

While animals host a huge number of pathogens, only a small number successfully jump to humans. A little over 200 zoonotic diseases have been identified to date (see overview). 

History 

The concept of zoonotic threats dates back to the 19th century when a famous German pathologist named Rudolf Virchow was studying a type of roundworm called Trichinella spiralis that infects and sickens humans when we eat contaminated, undercooked pork.

Driven by factors that elevate the risk of spillover from animals to humans—including population growth, land clearing, cultural practices, warming temperatures, factory farming, and more—scientists now estimate as much as 75% of new infectious diseases are zoonotic in origin. 

One of history’s deadliest and most well-known pandemics originated in animals—the bubonic plague. Caused by the bacteria Yersinia pestis and transmitted via flea bites, the bacteria has afflicted populations in recurring cycles for millennia. Most notably, it decimated roughly 40% of 14th-century Europe’s population.

Modern Examples 

The 20th century saw a number of exceptionally deadly global outbreaks driven by zoonotic viruses. 

The Spanish flu of 1918, which killed as many as 50 million people, is believed to have originated in birds. Separate waves caused by different strains of avian influenza occurred in 1957 and 1968, killing more than 1 million people each time. 

Human immunodeficiency virus, or HIV, originated in monkeys but later mutated into a human-only strain. It is thought to have jumped species when hunters were exposed to the infected blood of a subspecies of chimpanzee in west equatorial Africa.

Ebola, a devastating hemorrhagic disease (see 101), has seen sporadic but deadly outbreaks, mostly in sub-Saharan Africa where humans frequently come in contact with wild animals. Researchers think fruit bats are the likely reservoir for the Ebola virus.

Other high-profile examples include the coronavirus of 2020, rabies, bird flu, and many other viruses.

Transmission 

Transmission occurs in many ways—through direct contact, contaminated food or water, and more. In some cases, like bird flu, scientists are still uncertain about all the ways the disease may be transmitted between species.

A number of factors increase the likelihood of zoonotic spillover. Land clearing of forests in developing countries forces increased contact between animals forced from their habitat and humans. Markets where live animals are kept in jammed quarters awaiting sale are known transmission hot spots, while even researchers venturing into unexplored environments like cave systems may encounter new pathogens.

Most recently, the global COVID-19 pandemic was caused by a type of coronavirus that jumped from animals to humans, though the specific origin and route of transmission for SARS-CoV-2 was never definitively identified.

While the coronavirus family of viruses is quite large in animals (see overview), only seven known types have been identified in humans.

Why Certain Diseases Take Off 

There are thousands of diseases circulating in animals, and only a small fraction can spread to humans. What determines which are high-risk pathogens?

Viruses essentially pick cellular locks. Their spiky surfaces wedge themselves into keyhole-like receptors on cells allowing them to gain entry and make copies of themselves inside the cell (see overview). 

This replication process is quick and inexact so errors accrue—sometimes those mutations help the virus better fit into cellular receptors, boosting the viral load and ultimately enabling more transmission to occur.

More frequent contact between animals and people increases the risk of potential infections. But washing our hands frequently, properly cooking food, and checking for ticks can all help protect against zoonotic threats.