Wild birds can be affected by many different types of parasite or disease. In many cases these have relatively little impact on the individual, but in some cases they can cause illness or even death. Much of BTO's work in this area assesses the prevalence of disease in birds using gardens. Birds coming to feeders often congregate more closely than they would in the wild, so the risk of disease transmission can be increased. BTO works to understand how to minimise these risks, and to provide advice on the safest way to feed birds.