Humans use chemical pesticides to protect crops from insects. But the chemicals seep into rivers and can contaminate the food that wildlife eat.
The pesticides harm frogs' natural defences, making it easier for them to pick up infections from flatworms called trematodes.
So far scientists have seen frogs with extra legs in 43 different US states and all over Canada.
And they warn it could be a massive environmental problem for humans and wildlife if something isn't done.