Chagas disease is known for being fatal, but the disease is also known for being extremely hard to actually contract. This is due to the fact that to get infected a person needs to scratch the exact spot where a kissing bug has defected and bit them. The itching causes the parasite in the feces to enter into the skin. This process generally only results in about one of every 1,700 becoming infected.
Therefore, the guinea pigs continually get bite leading to their eventual infection. Once they become infected they become carriers and can pass the virus onto to those that eat it. Since humans have several large holidays that involve roasting a guinea pig in the region the end result is that they too end up infected.