"In the mid-1990s, people in the United States and Canada began to notice something grotesque. The frogs in their local ponds were sprouting extra legs"

Carl Zimmer:

As news of the deformed frogs spread, the Minnesota state government set up a hot line for sightings, and soon they got hundreds of calls from 54 out of 87 counties. “I’ve seen a lot of frogs over the years, and I’ve never seen anything like that,” a University of Minnesota herpetologist told the New York Times in 1996. 

Citizens and scientists alike feared that whatever was altering the frogs–pesticides perhaps–was also having an effect on humans. But researchers didn’t find any compelling link between frog deformities and humans diseases such as cancer. In fact, within a few years it looked as if the frogs were getting their legs naturally–through the manipulations of a parasite.