Spillover: Where Diseases Come From

Have you ever wondered where infectious diseases come from? Infectious diseases are caused by microbes, such as bacteria or viruses, and most of them are very contagious. Some microbes, like chlamydia, have evolved along side humans for hundreds of thousands of years. Some occur naturally in the environment, living in water or plants.

However, the majority of diseases which have caused the most trouble over the years, the ones which become global epidemics, or pandemics, are zoonotic diseases.


Zoonotic diseases are diseases that come animals. If humans come into contact with an animal population which has a high amount of disease, the infection can be transmitted from animals to humans. This phenomenon is called spillover, and it’s very common. Some examples of zoonotic diseases are ebola, salmonella, anthrax, bird flu, swine flu, lyme disease, SARS, rabies, zika, tuberculosis and nipah virus.. just to name a few. Even HIV was originally a disease that infected other primates. The animal species which the disease comes from are known as the reservoir species, and they can range from monkeys to bats, rats, pigs, cats, horses, birds and even whales! In fact, some diseases cross between multiple animal species before they get to humans, and this can make them especially dangerous.

The zoonotic Zika virus lies hidden in animals such as baboons. Image by Monica Brummit via Flickr

Bats, pigs and mutating DNA

One of the reasons that many zoonoses are dangerous diseases are because they spread very easily between humans are very difficult to contain. Most zoonotic viruses mutate very quickly. This means that their DNA changes spontaneously, making them able to infect multiple species. In 1999, an outbreak of Nipah virus in Malaysia caused 105 deaths. Nipah virus causes a condition known as encephelitis, which is the swelling of the brain, and can be very deadly. Scientists were able to look at the DNA of the virus and figure out that the reservoir species were flying foxes, a type of fruit bat. However, humans had very little contact with these bats, and hardly any of the bats lived near cities.

It turns out that there was an intermediate animal host between the bats and humans – pigs. The bats were eating fruit near pig farms, and their droppings were infecting the pigs. Because the pigs were exposed to the virus, the virus was able to mutate to become similar to the pigs DNA. Scientists believe that because pig DNA is quite similiar to human DNA, this allowed the virus to spillover into humans more easily. The outbreak was controlled by removing all fruit trees near farms, and closing down all pigs farms in the country.

Flying foxes were the reservoir species of the 1999 Nipah outbreak which killed more than 100 people. Image by Stephen Patterson via Flickr

One health: predicting the next epidemic

So why are zoonotic viruses so important? Well, many scientists believe that the next big pandemic will be caused by a zoonotic virus. They are extremely difficult to predict and can cause outbreaks almost immediately after being transferred to humans. Scientists are constantly monitoring countless animal species and the viruses they contain. There is a lot of research being conducted to try and predict which types of animals are most likely going to be the reservoir for the next human pandemic. Many infectious disease experts are now part of a collaborative effort known as one healthThe one health philosophy argues that there is a complex interaction between human health, animal health and the health of ecosystems and the environment. Only by understanding and caring for all three will we be able to truly predict and control the next spillover epidemic.

Many scientists are still studying the 2014 Ebola outbreak for clues on how the next pandemic will spread. Image by tony.cairns via Flickr