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Also Known as: transmissible disease or communicable disease


Definition

By Mayo Clinic Staff

  • Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies. They're normally harmless or even helpful, but under certain conditions, some organisms may cause disease.
  • Some infectious diseases can be passed from person to person. Some are transmitted by bites from insects or animals. And others are acquired by ingesting contaminated food or water or being exposed to organisms in the environment.
  • Signs and symptoms vary depending on the organism causing the infection, but often include fever and fatigue. Mild complaints may respond to rest and home remedies, while some life-threatening infections may require hospitalization.
  • Many infectious diseases, such as measles and chickenpox, can be prevented by vaccines. Frequent and thorough hand-washing also helps protect you from infectious diseases.

Causes

  • Bacteria. These one-cell organisms are responsible for illnesses, such as strep throat, urinary tract infections, Cholera, Diarrhea and Tuberculosis.
  • Viruses. Even smaller than bacteria, viruses cause a multitude of diseases — ranging from the common cold to AIDS and include Influenza and Smallpox.
  • Fungi. Many skin diseases, such as ringworm and athlete's foot, are caused by fungi and are sometimes difficult to cure, but are not considered dangerous. Other types of fungi can infect your lungs or nervous system system such as coccidioidomycosis, sometimes called valley fever, a lung infection that is prevalent in the SW United States. Cryptococcosis is another fungus disease that may be localized in the lung or disseminated, especially to the central nervous system. Histoplasmosis can cause ulcers of the pharynx and enlargement of the liver and spleen, or tubercularlike lesions of the lungs..
  • Parasites. Malaria is caused by a tiny parasite that is transmitted by a mosquito bite. Other parasites may be transmitted to humans from animal feces.

Direct contact

An easy way to catch most infectious diseases is by coming in contact with a person or animal who has the infection. Three ways infectious diseases can be spread through direct contact are:

  • Person to person. A common way for infectious diseases to spread is through the direct transfer of bacteria, viruses or other germs from one person to another. This can occur when an individual with the bacterium or virus touches, coughs on or kisses someone who isn't infected.
 These germs can also spread through the exchange of body fluids from sexual contact or a blood transfusion. The person who passes the germ may have no symptoms of the disease, but may simply be a carrier.
  • Animal to person. Being bitten or scratched by an infected animal — even a pet — can make you sick and, in extreme circumstances, can be fatal. Handling animal waste can be hazardous, too. For example, you can acquire a toxoplasmosis infection by scooping your cat's litter box.
  • Mother to unborn child. A pregnant woman may pass germs that cause infectious diseases to her unborn baby. Some germs can pass through the placenta. Germs in the vagina can be transmitted to the baby during birth.

Indirect contact

Disease-causing organisms also can be passed by indirect contact. Many germs can linger on an inanimate object, such as a tabletop, doorknob or faucet handle. When you touch a doorknob handled by someone ill with the flu or a cold, for example, you can pick up the germs he or she left behind. If you then touch your eyes, mouth or nose before washing your hands, you may become infected.

Insect bites

Some germs rely on insect carriers — such as mosquitoes, fleas, lice or ticks — to move from host to host. These carriers are known as vectors. Mosquitoes can carry the malaria parasite or West Nile virus, and deer ticks may carry the bacterium that causes Lyme disease.

Food contamination

Another way disease-causing germs can infect you is through contaminated food and water. This mechanism of transmission allows germs to be spread to many people through a single source. E. coli, for example, is a bacterium present in or on certain foods — such as undercooked hamburger or unpasteurized fruit juice.

Some Examples:

Historic pandemics

A pandemic (or global epidemic) is a disease that affects people over an extensive geographical area.

  • Plague of Justinian, from 541 to 750, killed between 50% and 60% of Europe's population.[38]
  • The Black Death of 1347 to 1352 killed 25 million in Europe over 5 years. The plague reduced the world population from an estimated 450 million to between 350 and 375 million in the 14th century.
  • The introduction of smallpox, measles, and typhus to the areas of Central and South America by European explorers during the 15th and 16th centuries caused pandemics among the native inhabitants. Between 1518 and 1568 disease pandemics are said to have caused the population of Mexico to fall from 20 million to 3 million.[39]
  • The first European influenza epidemic occurred between 1556 and 1560, with an estimated mortality rate of 20%.[39]
  • Great Plague of Marseille in 1720 killed 100,000 people in the city and the surrounding provinces
  • Smallpox killed an estimated 60 million Europeans during the 18th century[40] (approximately 400,000 per year).[41] Up to 30% of those infected, including 80% of the children under 5 years of age, died from the disease, and one-third of the survivors went blind.[42]
  • In the 19th century, tuberculosis killed an estimated one-quarter of the adult population of Europe;[43] by 1918 one in six deaths in France were still caused by TB.
  • The Influenza Pandemic of 1918 (or the Spanish Flu) killed 25-50 million people (about 2% of world population of 1.7 billion).[44] Today Influenza kills about 250,000 to 500,000 worldwide each year.

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