Did you know that amphibians have very special skin? They use their skin to breathe and drink water. But a skin-eating fungus, Batrachochytrium dendrobatidis (Bd), is killing them. Since the 1970s, over 200 species of amphibians have declined or gone extinct. Amphibians in the eastern US seem to be unaffected by Bd, but Bd outbreaks have caused mass die-offs in the western US. A frog species native to the eastern US, American bullfrogs (Rana catesbeiana), may have helped spread Bd. Bullfrogs don’t show signs of sickness when they are infected, which makes them Bd vectors. This is alarming because they are traded alive globally and could continue spreading Bd to amphibians around the world. Here, we analyzed the history of bullfrogs and Bd in the western US. We found a link between bullfrogs’ arrival and Bd outbreaks. Then we predicted areas with high disease risk. These results can help us control the spread of Bd and save amphibians.