Fungi are large organisms that usually live on dead and rotting animal and plant matter. They are found mostly in soil, on objects contaminated with soil, on plants and animals, and on skin, and they may also be airborne. Fungi may exist as yeasts or molds and may alternate between the two forms, depending on environmental conditions. Yeasts are simple cells, 3 to 5 micrometres (0.0001 to 0.0002 inch) in diameter. Molds consist of filamentous branching structures (called hyphae), 2 to 10 micrometres in diameter, that are formed of several cells lying end to end. Fungal diseases in humans are called mycoses; they include such disorders as histoplasmosis, coccidioidomycosis, and blastomycosis. These diseases can be mild, characterized by an upper respiratory infection, or severe, involving the bloodstream and every organ system. Fungi may cause devastating disease in persons whose defenses against infection have been weakened by malnutrition, cancer, or the use of immunosuppressive drugs. Specific types of antibiotics known as antifungals are effective in their treatment.
Asthma is associated with rhinovirus, human respiratory syncytial virus, and the bacterium Chlamydia pneumoniae. Chlamydia pneumoniae is particularly associated... more