Anyone who has ever interacted with a dog knows about the species’ proclivity to lick. Licks are thought of, at most, as sloppy so recent headlines reporting the story of a Wisconsin man who contracted a fulminant limb-destroying infection from a dog’s lick are likely puzzling to the general public.

How did this rare infection occur? The 48-year-old man hospitalized in this case developed an infection with a bacterial species called Capnocytophaga canimorsus . This bacterial species is highly prevalent in the mouths of both dogs and cats and isn’t harmful to them, or most humans. However, certain individuals, when exposed to the bacterium through a lick, or more frequently a bite, can develop aggressive infections that are often life-threatening. For a lick to cause infection, the bacteria has to find its way into the body through an opening such as a wound or abrasion. A few hundred cases of Capnocytophaga occur annually in the United States (while countless numbers of dog licks occur per hour).

It is important to emphasize that damaging infections like this are a complex interplay between a microbe and the specific immune system of the individual involved. Immune responses that are either underwhelming or over exuberant both can result in severe symptoms.