Health of Frogs
Frogs (and Toads, but mainly Frogs) are what's known as an "Indicator Species".
Because they spend so much of their time in and around the water, they are a great indicator of how clean the water is.
Changes in temperature, changes in water chemistry can affect frogs drastically. Frogs absorb a lot of water through their skin. If pollutants are present in the water, such as runoff from farm fields, chemical leaks from factories, etc.), the frogs will absorb these, too.
All these chemical inputs can have drastic effects on how frogs develop, and can cause severe deformities.