Sarcoidosis is a multisystem disease, characterized by the presence of noncaseating granulomas in various organs. It generally affected the lungs (in more than 90% of the cases), but can have a large variety of symptoms.
Its causes are unknown. There are, however, some possible associations with the disease, among them being infectious agents (tuberculosis, other mycobacterial infections) and immunological defects (patients tend to present an impaired immune response).
The clinical presentation varies greatly and depends on what areas of the body are affected. If the lungs are affected, it can present with fatigue, shortness of breath, chest pain and epistaxis (nosebleed). It can also present with uveitis (an eye infection that may progress to blindness), a variety of skin lesions, joint compromise (polyarthiritis), nervous system compromise, heart problems (cardiomyopathy) and liver function abnormalities.
To many, it's become a sort of "in joke" that sarcoidosis (and/or in many episodes, Lupus) is mentioned or suggested as a possible diagnosis in such a large number of House episodes, if only briefly, before being dismissed.