Leeches may be gross, but their blood-feeding habits and painkilling saliva make them useful tools for medicine all over the world. And according to a new study, these slimy suckers could have yet another service to offer humanity: helping scientists track the health of rare rainforest animals. After a leech feeds, it stores its victim’s blood for months—an ability researchers have already used to look for elusive mammals like the Truong Son muntjac (Muntiacus truongsonensis), a tiny forest-dwelling deer from Vietnam. And more recently, some of the same team decided to test whether leeches store signs of not just the presence of their victims, but also the diseases they carry. The science isn’t for the squeamish. The researchers filled pig intestines with human blood that had been spiked with four different viruses—including a type of herpes—and let medicinal leeches (Hirudo spp.) suck on the resulting blood bubbles. After dissecting the leeches and amplifying the genetic material found within, the team still detected three of the four viruses 50 days later, they report in the European Journal of Wildlife Research. That means leeches in the wild may hold signs of the health of their victims weeks after they feed, as long as they store genetic material in the same way as their medicinal relatives. The research could provide an easier alternative to tracking down diseased wildlife that live in hard-to-access habitats, the team says. After all, finding the wild leeches isn’t too challenging: Some species happily seek out intrepid researchers, eager for the chance at a fresh meal.