A strain of fungal skin pathogen (Trichophyton benhamiae var. luteum) is spreading rapidly among guinea pigs and people in Europe, but scientists didn’t understand why it was more contagious than closely related strains. Researchers compared gene activity in four related fungal species and found that the epidemic strain produces higher levels of toxic compounds called secondary metabolites. These compounds help the fungus escape the body’s immune system and cause infection more effectively than in less dangerous relatives.