Pneumonia

pneumonia, inflammation and consolidation of the lung tissue as a result of infection, inhalation of foreign particles, or irradiation. Many organisms, including viruses and fungi, can cause pneumonia, but the most common causes are bacteria—in particular, species of Streptococcus and Mycoplasma.

Although viral pneumonia does occur, viruses more commonly play a part in weakening the lung, thus inviting secondary pneumonia caused by bacteria. Fungal pneumonia can develop very rapidly and may be fatal, but it usually occurs in hospitalized persons who, because of impaired immunity, have reduced resistance to infection. Contaminated dusts, when inhaled by previously healthy individuals, can sometimes cause fungal lung diseases. Pneumonia can also occur as a hypersensitivity, or allergic response, to agents such as mold, humidifiers, and animal excreta or to chemical or physical injury (e.g., smoke inhalation).