Because studying the transmission of typhus required that the parasite be alive (needed a human host), scientists were only able to study it during epidemic times. However, Nicolle found that the chimpanzee served as a suitable alternate host for this study since it was genetically similar to humans, and in June 1909, Nicolle tested his theory by infecting a chimpanzee with typhus, retrieving the lice from it, and placing it on a healthy chimpanzee. Within 10 days, the second chimpanzee had typhus as well. After repeating his experiment, he was sure of it: lice were the carriers. As Nicolle continued his ongoing research on the disease, he later switched to using guinea pigs as his model organism instead of chimpanzees as they were just as susceptible to infection and were also smaller and cheaper.