The Diagnosans appear in the episodes Die Me, Dichotomy (Season 2), Season of Death (Season 3), and The Choice (Season 3), as well as the Farscape: The Peacekeeper Wars miniseries.
Due to their natural ability to detect and treat a variety of diseases in all types of living things, many of them become physicians. They have complex minds and an equally complex language, so much so that standard translator microbes cannot decode the intricacies of their speech. They are soft-spoken and gentle, further adding to their abilities as doctors. Diagnosans can diagnose disease in many different races, from humans to Leviathans, such as Moya.
Their primary flaw as a species is, ironically, their own inability to tolerate infection in their bodies. Typically, Diagnosans wear special masks whenever they are in a situation where they may come into contact with a contagion. Usually, they use a sterile force field when performing surgery, but they must put their mask back on before turning off the field, since even traces of a disease can prove fatal if inhaled into their nose and mouth at the same time.
Read more about this topic: List Of Races In Farscape