Psittacosis is a serious disease acquired by handling birds or by inhaling dust from bird faeces. It is caused by the bacterium Chlamydia psittaci , an obligate intracellular parasite. After entering the respiratory tract, the cells are transported to the liver and spleen, multiply there and then invade the lungs, causing inflammation, haemorrhage and pneumonia. Legionnaire's disease is a fairly common form of pneumonia in older or immunocompromised people. It is seldom transmitted directly from person to person.
Infection occurs when people inhale aerosol droplets containing the bacteria.
Acute allergic alveolitis is a serious hypersensitive response, usually associated with repeated exposure to airborne spores in the work environment. A classic example is the condition termed farmer's lung , caused by exposure to spores of thermophilic actinomycetes. Aspergillosis , Histoplasmosis and Coccidioidomycosis are examples of serious fungal infections of humans, initiated by spores deposited in the alveoli. They can be life-threatening diseases of immunocompromised people, when the fungi disseminate from the lungs to major organs of the body. However, in all cases the infection of humans is incidental to the fungus, playing no part in its normal biology.