Infection

The establishment in the body of disease-causing microorganisms (such as bacteria, viruses, or fungi). The organisms reproduce and cause disease by direct damage to cells or by releasing toxins. This normally provokes the immune system into responding, which accounts for many common symptoms. Infection can be localized within a particular area or tissue, as in a boil, or be systemic (spread throughout the body), as in influenza. Weakness, aching joints, and fever are expressions of systemic infectious disease. Localized infection may result from the spread of organisms through wounds, or during surgery. Localized infection is generally associated with pain, redness, swelling, and formation of a pus-filled abscess at the site of infection, and a rise in temperature. Many minor infections are dealt with by the immune system and need no specific treatment. Severe systemic infections may need treatment with drugs such as antibacterials or antivirals. A localized infection that has produced pus may be drained surgically.