The Controlled Human Infection Model or CHIM, sometimes described as a human challenge study, is a relatively specialised medical research technique. Researchers infect healthy participants with a weakened strain of a pathogen in a controlled setting, in order to learn more about the infection and the disease, or to develop new vaccines for that disease. Unlike in other human clinical trials, where participants face a risk of harm because of, for example, the drug's side effects, healthy participants in CHIM trials are deliberately harmed through infection – contrary to every principle and guideline of medical practice and research.