Contamination through the insect's feces is the primary mechanism by which vinchucas pass T. cruzi to humans. After the bugs blood feed, they defecate and deposit the metacyclic trypomastigote parasite onto the skin. This tryp travels from the skin to the victim's blood via the bite site. The site is scratched and the parasite is rubbed into the wound. The metacyclic trypomastigotes circulate for a short while in the blood before they enter tissue and become encysted as amastigotes. Amastigotes produces stubby trypomastigates that enter the bloodstream and get picked up by other vinchucas when they bloodfeed, thus sterile vinchucas become infected.

Not all species of triatomines defecate at the same rate: T. infestans defecate 30% of the time and soon after ingestion. This makes this species particularly infective for humans. Those species that defecate closest to the wound are most likely to transmit the parasite. Children are most vulnerable to being contaminated because they spend a lot of time around the house and wear little clothing, making them especially susceptible to contaminating themselves with fecal deposits throughout the house.