Infectious diseases are caused by microorganisms that invade the body and multiply. These microorganisms can include bacteria, viruses, parasites, and fungi. Normally, the body's natural defenses prevent the microorganism from causing illness. Oftentimes, however, the microorganisms resist the body's defenses and cause infection. The severity of infections can range from mildly annoying, such as with a cold or diarrheal illness, to life threatening, as with meningitis or AIDS. Severity of the infection depends on the overall health of the patient and virulence (strength) of the microorganism.

Show Full Description Infectious disease causes about one-fourth of the world's 55 million deaths each year. About 90% of infectious disease deaths are caused by influenza, pneumonia, diarrheal diseases, tuberculosis, malaria, measles, hepatitis B, and HIV/AIDS. There are many ways that infectious disease can be spread. Most commonly, infectious diseases are spread indirectly. Touching an object, such as a faucet, that an infected person has touched and then touching your eyes, mouth, or nose before washing your hands can spread infection. Other infectious diseases are spread through the air (airborne), for example, microorganisms can be carried in the droplets of moisture when someone who is already infected coughs or sneezes. Direct contact, as through exchange of body fluids with an infected person (for example, through sexual contact or blood transfusion), is another route of transmission. Some microorganisms, such as HIV, can pass through the placenta or breast milk, enabling transmission of disease from mother to baby. Infectious disease can also be spread from the bite or sting of an infected insect or by being bitten or scratched by an infected animal. Eating contaminated food can cause disease as well. Certain factors influence the body's susceptibility to infection: a weakened or compromised immune system can impair the body's natural defenses. Conditions that can affect a person's immune system include undergoing surgery; taking certain drugs; having a disease that weakens the immune system, such as having AIDS; or being very young or elderly. Through worldwide efforts, many infectious diseases have been brought under control or eliminated. Unfortunately, newer diseases are emerging. And some infectious diseases that were becoming less common have surged back. Overuse of antibiotics has made many bacteria resistant to medication; once treatable infections now have the potential to develop into life-threatening illnesses. People can help prevent infection through frequent hand washing, taking antibiotics only when necessary, keeping cooking surfaces clean and refrigerating foods, practicing safe sex, and following healthcare recommendations for infectious disease prevention. The information provided in the Infectious Disease Condition Guide is not intended to be a substitute for professional medical advice. A licensed healthcare professional should be consulted for diagnosis of any infectious disease and treatment of any and all medical conditions.