Two campers in Yosemite National Park were infected with hantavirus in June, and one has died, public health officials announced last week. The pair contracted the rare disease after coming into contact with infected mouse droppings. Mice carry hantavirus. Mosquitoes carry malaria. Dogs carry rabies. What animals cause the most human disease?

Cows, pigs and chickens. Some animal-borne illnesses garner media coverage because their symptoms and mortality rates are terrifying. Ebola, for example, causes severe internal and external bleeding and kills between 25 and 90 percent of its victims. Other diseases, like anthrax, are known for their potential as weapons of bioterrorism.

But most of the illnesses we contract from animals are the decidedly less newsworthy diseases of the gastrointestinal tract. According to a study released in July, 2.3 billion people contract diarrheal illnesses annually from animals, usually from eating infected meat, eggs or dairy products. That’s more than 10 times the number of people who get malaria each year from mosquito bites.

Malaria kills a higher percentage of its victims, but the annual number of deaths from zoonotic gastrointestinal disease overall is still more than double that of malaria.

Toss in a handful of other barnyard diseases, like mad cow and brucellosis, and livestock are far and away the most disease-bearing animals from a human perspective.

The danger of mosquitoes reaches beyond malaria. Dengue infects 50 million people per year, while around 500,000 people come down with chikungunya. Mosquitoes also carry a host of viruses that infect the brain and cause encephalitis, from Eastern equine to West Nile. (The latter is responsible for the current state of emergency in Dallas.)

Rodents also carry an impressive diversity of diseases that can infect humans. Hantavirus infects 175,000 people and kills about 1,750 per year, but isn’t the biggest threat. Leptospirosis infects about 1.7 million people per year and kills more than 100,000 of them after bouts of fever, muscle aches, vomiting, and kidney or liver failure. (Livestock are partially to blame for leptospirosis as well, because cattle and pigs can also carry it in their urine.) Rats, squirrels, and other rodents also carry Yersinia pestis, better known as plague, which still infects between two and 10 people in the United States each year. The straightforwardly titled rat-bite fever is another rodent-borne illness with global reach.

Birds are a somewhat special case. Consider, for example, avian influenza. In terms of impact on human health, it’s not particularly significant, infecting between 100 and 200 people in an average year. As an emerging disease, however, it’s of the highest concern. If avian influenza mutates to a form that can be passed from person to person, it would cease to be an animal-borne illness and instantly become a major public health crisis.

Primates also occupy a unique category as disease reservoirs. We don’t have a lot of contact with our fellow apes, which is probably a good thing because our close evolutionary relationship puts us at special risk for contracting their diseases. When a bug successfully makes the jump from ape to human, it can be catastrophic. HIV, for example, which evolved from a chimpanzee virus, has killed approximately 30 million people since the epidemic began. Primates carry a variety of other zoonotic diseases, such as the rare but deadly herpes B, monkeypox and Yaba virus.


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Explainer thanks Bruno Chomel of the University of California-Davis School of Veterinary Medicine.