A deer tick climbs a blade of tall grass in the woods. It stretches its front legs as far as it can, waiting for a passerby. When a hiker brushes past, the tick clings on. It finds a prime position, cuts into the skin, and inserts its feeding tube. If the tick carries the bacteria that causes Lyme disease, that virulent microbe passes into the tick’s human host, and the person now has a zoonotic disease.

Also called zoonoses, zoonotic diseases are the many kinds of illnesses that pass from animals to humans. Almost 16 percent of all deaths worldwide can be attributed to infectious diseases, and zoonoses account for 60 percent of known infectious diseases and 75 percent of emerging infectious diseases, according to the National Institutes of Health.

Zoonoses can be transmitted in many ways, including animal and insect bites; petting or otherwise handling sick animals; and consuming undercooked meat, unpasteurised milk, or contaminated water. The types of pathogens that can be transmitted by animals to humans include bacteria, parasites, fungi, and viruses.

Some zoonotic diseases are relatively benign, but many—for example, Lyme disease—are quite harmful. Some, such as the coronavirus that causes COVID-19, can be deadly.

The leading theory on the origin of the novel coronavirus pandemic is that the virus made the leap into humans at a wildlife market in Wuhan, China, in late 2019. It’s not precisely known how the virus moved from its original host to the wildlife market and on to people. But because humans have no immunity to the virus, COVID-19 to date has sickened three quarters of a million people and killed tens of thousands worldwide. (Follow all of our coronavirus coverage here.)

Another well-known zoonotic disease, Ebola virus, is thought to have originated in fruit bats, which still act as carriers out in the wild, as do apes. Bats and apes can pass the virus to humans through direct contact or if people eat their meat. Other zoonoses travel from a wild host species to domesticated animals—say, from bats to farmed pigs. Humans can then get sick from handling and eating the pigs.

Common sources of zoonotic disease

Most people live around animals, even if they don’t have farms or pets. From ticks to squirrels to rats, many members of the animal kingdom can transmit disease. Here’s a short list of common animals and some of the diseases we can get from them:

Cats: toxoplasmosis; Pasteurella; ringworm

Bats: Ebola virus; SARS; MERS; rabies; Nipah virus; Hendra virus

Dogs: rabies; noroviruses; Pasteurella; salmonella; ringworm; hookworm

Ticks: Lyme disease; Rocky Mountain spotted fever; Powassan disease

Mosquitoes: malaria; dengue; West Nile virus; Zika virus; Chikungunya virus

Birds: bird flu (H1N1, H5N1); salmonella; psittacosis

Cows: Escherichia coli; ringworm; salmonellosis

Rodents: Hantavirus pulmonary syndrome; plague; rat-bite fever; salmonellosis