Chagas disease is fatal, but hard to catch. In order for people to become infected, they need to scratch the spot where a bloodsucking insect known as the kissing bug has both bit them and defecated on them. The itching scratches the feces—loaded with a single-celled parasite known as Trypanosoma cruzi—into the skin, but the process is so inefficient that only one human becomes infected for every 1700 kissing bug bites. So why are nearly 40% of people in some South American communities infected with T. cruzi? At least in Arequipa, Peru, the answer may be guinea pigs, according to a study published online today in the Proceedings of the Royal Society B. The rodents, which are raised as food in the Andean region, can act as reservoirs of T. cruzi, infecting bugs that hang around their pens and suck their blood. If there were lots of uninfected guinea pigs to balance out their infected friends, this might not be such a problem. But guinea pig populations rise and fall in a predictable cycle: When alfalfa prices spike in the dry summer months, people kill their stock rather than pay more to feed them. (It doesn’t hurt that summer contains several holidays that traditionally involve large guinea pig cookouts in the Andes.) If the remaining guinea pigs happen to carry T. cruzi, the parasite becomes concentrated in a smaller population. That, in turn, makes it more likely a hungry kissing bug will bite an infected guinea pig instead of a healthy one, turning the insect into a vector capable of passing along T. cruzi to its future meals, including humans. And that appears to be exactly what’s happening in Arequipa. After many years of this cycle, more than 80% of the kissing bugs taken from two guinea pig pens there were infected with T. cruzi, whereas only about 6% of insects sampled away from the rodent enclosures were. Unfortunately, the bugs that live close to guinea pigs also live close to people, increasing the chance that T. cruzi will find its way into human hosts and keep the Chagas rate high.

*Correction, 18 June, 1:26 p.m.: This article originally referred to the kissing bug as a species of beetle, which is incorrect. It belongs to the family of insects called Reduviidae.