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What Is, Scientifically, the Grossest Animal?

As measured by the ability to kill via infectious disease.
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Cute, yes. But also a dirty, dirty animal. (Photo: likeaduck/Flickr)

Cute, yes. But also a dirty, dirty animal. (Photo: likeaduck/Flickr)

The plague is back. A second visitor to Yosemite National Park has a confirmed case of plague, the same infection that killed millions in Europe during the Middle Ages, according to the Los Angeles Times. The likely disease carriers in this case: squirrels and chipmunks.

Although rare and mostly curable today, plague's persistence is a reminder that people have good reason to find certain animals—such as rats, whose fleas spread plague in medieval times—gross. But which animal is really the grossest? That is, which is carrying the deadliest infectious diseases?

To answer that, we must look to the data:


  • What illnesses can they give you? So many.
  • How common are the illnesses? Some, like plague, are very rare. In recent decades, the United States has had an average of seven plague cases a year, according to the Centers for Disease Control and Prevention. But consuming rat-feces-contaminated food or drink can also give you salmonella, which is a pretty common illness, compared to the other diseases we'll discuss here. About 40,000 cases of salmonella are reported in the U.S. every year. But because people likely don't report mild cases, the true number may be more than one million, according to the CDC. It's not clear how many salmonella cases are due to rats instead of other vectors, such as contaminated eggs.
  • How deadly are the illnesses? Eleven percent of people who have contracted plague recently in the U.S. have died. The mortality rate for salmonella, on the other hand, is around one in three million.
  • Grossness factor: Nine out of 10. Although no longer deadly, it's clear humanity's shuddering reaction to rats is well-deserved.


  • What illnesses can they give you? They're not usually the direct cause of infectious diseases, although they "may play a supplemental role in the spread of some diseases" such as diarrhea and typhoid fever, according to the World Health Organization. Cockroaches also trigger allergies and asthma in some folks, but those are chronic, not infectious, diseases.
  • How common are the illnesses? Americans get millions of episodes of diarrhea per year. Several thousands of Americans get typhoid fever a year, but mostly while traveling abroad. It's unknown how many of these cases have anything to do with cockroaches.
  • How deadly are the illnesses? In the U.S., 300 or 400 children die a year from diarrhea, according to an estimate published in 1991. Between 1999 and 2006, a little more than one-tenth of one percent of Americans who contracted typhoid died.
  • Grossness factor: Technically, because they don't directly cause infectious disease, cockroaches shouldn't score highly on our grossness scale. The World Health Organization, however, says cockroaches can feed on "the fingernails and toenails of babies and sleeping or sick persons," which is worth at least three points, so let's say five.


  • What illnesses can they give you? Plague and tularemia, among others. Yosemite staff found plague in the fleas carried on squirrels and chipmunks, traditionally considered cute animals. Tularemia is a fever-causing illness that the CDC considers to be a potential bioterrorism weapon.
  • How common are these illnesses? We discussed plague above. About 200 people a year get sick from tularemia in the U.S.
  • How deadly are the illnesses? Thirty to 60 percent of people infected with tularemia die.
  • Grossness factor: Eight. Surprisingly gross, due mostly to the high fatality rate of tularemia.


  • What illnesses can they give you? Salmonella. Avian flu.
  • How common are these illnesses? Salmonella is common, as we discussed. Both backyard and commercially raised chickens get avian flu from time to time and, this year, outbreaks have forced farmers to cull so many birds, it has affected egg prices in the U.S. In the last 20 years, however, only a handful of people have contracted avian flu in North America.
  • How deadly are the illnesses? No one has died of avian flu in the U.S., but deadly epidemics have occurred in Asia.
  • Grossness factor: Salmonella is common and gross, but luckily not normally fatal in North America. Let's go with an innocuous two.


  • What illnesses can they give you? Many, including gut, eye, and skin infections.
  • How common and deadly are these illnesses? We've discussed the prevalence of some of the diseases flies transmit, such as diarrhea and typhoid, above. In general, besides diarrhea, the infections flies transmit are rare in the U.S.
  • How deadly are the illnesses? The diseases flies transmit rarely kill when treated in the U.S.
  • Grossness factor: Three. Flies transmit many gross diseases, but most of them are not deadly in the U.S., which depresses their score, on our scale.

And the winner is ... rats! Just another reason to stay well away from the edge of the platform in subway stations.

Sanitary systems in the U.S. mean that most traditionally gross animals, such as rats and flies, are no longer as dangerous as they once were—but still are, in developing countries and sometimes even in poorer regions of the U.S. Still, it's wise to be wary of animals, both wild and domestic, and give your hands a good wash after coming into contact with them.