"He’ll make a wart on you"
The grey tree frog is common across North America. Its wart-like glands are clearly visible in this picture.
Creative Commons Attribution Share AlikeThe grey tree frog is common across North America. Its wart-like glands are clearly visible in this picture. - Credit: cotinis
Touching frogs allegedly gives you warts
Creative Commons AttributionTouching frogs allegedly gives you warts - Credit: Bmann

This folk belief is generally applied to toads rather than frogs, although in fact there is no taxonomic distinction between the two categories. It derives from the resemblance of the amphibian’s parotoid glands — which are located on the back, neck and shoulders and which secrete an alkaloid substance that deters predators — to warts.