One human’s harmless virus can be another animal’s worst nightmare, as a recent report well illustrates. Veterinary researchers at the University of Florida have confirmed the first cases of herpes simplex virus type 1 (HSV-1) in lemurs, one of which may have been infected via a shared yoga class. Though the virus typically only causes mild cold sores in people, it proved fatal for all of the lemurs they tried to treat. The findings highlight the importance of being careful in how people interact with wildlife, the researchers say, especially animals that aren’t naturally immune to the viruses we carry. “We should recognize that we can be dangerous,” senior study author Jim Wellehan, a professor at the University of Florida’s College of Veterinary Medicine, told Gizmodo. Deadly lemur herpes HSV-1 (also formally known as Human alphaherpesvirus 1) has been bumming a ride on humans for quite some time. Research has suggested that it first started infecting our direct ancestors about six million years ago. Today, about two-thirds of people under 50 worldwide are thought to carry HSV-1. And though it can be a life-threatening infection for certain groups, like infants, or an unseemly issue when it causes genital herpes, most people infected with HSV-1 will never experience anything more serious than cold sores.