Building-related illnesses are those for which there is a clinically defined illness of known etiology. These include infections such as legionellosis and allergic reactions such as hypersensitivity diseases and are often documented by physical signs and laboratory findings. Building-related illnesses are uncommon and are more serious in prognosis than mere discomfort. Physician diagnosis by clinical investigation of symptoms is the usual means of recognizing them. These illnesses can have a long latent (or asymptomatic) period after exposure begins before symptoms are experienced, similar to lung cancer after indoor radon exposure. Other categories of building-related illnesses, however, are associated with an immediate appearance of symptoms after exposure. They include toxic illness such as carbon monoxide poisoning; infectious diseases such as Legionella or Legionnaires' disease; and allergic disease such as asthma, hay fever, or hypersensitivity pneumonitis.