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Causes
By Mayo Clinic staffInfectious diseases can be caused by:
- Bacteria. These one-cell organisms are responsible for such illnesses as strep throat, urinary tract infections and tuberculosis.
- Viruses. Even smaller than bacteria, viruses are the cause of a multitude of diseases — ranging from the common cold to AIDS.
- Fungi. Many skin diseases, such as ringworm or athlete's foot, are caused by fungi. Other types of fungi can infect your lungs or nervous system.
- 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 someone who has one. This "someone" can be a person or an animal or for an unborn baby, its mother. Three different ways infectious disease can be spread through direct contact are:
- Person to person. The most 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.
- Animal to person. Your household pet might seem harmless, but pets can carry many germs. Being bitten or scratched by an infected animal can make you sick and, in extreme circumstances, could even cause death. 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 the birthing process.
Indirect contact
Disease-causing organisms can also be passed along by indirect contact. Many germs can linger on an inanimate object, such as a tabletop, doorknob or faucet handle. When you touch the same doorknob grasped 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 is a bacterium present in or on certain foods — such as undercooked hamburger or unwashed fruits or vegetables.
- Understanding microbes in sickness and in health. National Institute of Allergy and Infectious Diseases. http://www3.niaid.nih.gov/topics/microbes/PDF/microbesbook.pdf. Accessed May 6, 2009.
- Facts about infectious diseases. Infectious Diseases Society of America. http://www.idsociety.org/factsaboutid.html. Accessed May 6, 2009.
- Opal SM, et al. Host responses to infection. In: Cohen J, et al. Cohen and Powderly: Infectious Diseases. 2nd ed. Philadelphia, Pa.: Mosby Elsevier; 2004. http://www.mdconsult.com/das/book/body/135757989-3/0/1209/7.html?tocnode=49351280&fromURL=7.html#4-u1.0-B0-323-02407-6..50004-0_85. Accessed May 6, 2009.
- Iacoviello VR, et al. Principles of anti-infective therapy. In: Cohen J, et al. Cohen and Powderly: Infectious Diseases. 2nd ed. Philadelphia, Pa.: Mosby Elsevier; 2004. http://www.mdconsult.com/das/book/body/135757989-5/0/1209/535.html?tocnode=49356432&fromURL=535.html#4-u1.0-B0-323-02407-6..X5001-6--section7_5237. Accessed May 6, 2009.
- Steckelberg J (expert opinion). Mayo Clinic, Rochester, Minn. May 18, 2009.
- Routes of transmission. In: Mandell GL, et al. Mandell, Douglas and Bennett's: Principles and practice of infectious diseases. 6th ed. Philadelphia, Pa.: Elsevier; 2006. http://www.mdconsult.com/das/book/body/136228628-6/838936308/1259/99.html#4-u1.0-B0-443-06643-4..50015-5--cesec23_433. Accessed May 8, 2009.
- Crowcroft N, et al. Prevention. In: Cohen J, et al. Cohen and Powderly: Infectious Diseases. 2nd ed. Philadelphia, Pa.: Mosby Elsevier; 2004. http://www.mdconsult.com/das/book/body/135757989-5/0/1209/9.html?tocnode=49351308&fromURL=9.html#4-u1.0-B0-323-02407-6..50005-2_113. Accessed May 6, 2009.
- Johnson KS, et al. Lumbar puncture: Technique, indications, contraindications and complications in adults. http://www.uptodate.com/home/index.html. Accessed May 8, 2009.
- Chest X-ray (chest radiography). Radiological Society of North America. http://www.radiologyinfo.org/en/info.cfm?PG=chestrad. Accessed May 13, 2009.
- CT: Body. Radiological Society of North America. http://www.radiologyinfo.org/en/info.cfm?pg=bodyct. Accessed May 13, 2009.
- MRI of the head. Radiological Society of North America. http://www.radiologyinfo.org/en/info.cfm?PG=headmri. Accessed May 13, 2009.
- An ounce of prevention keeps the germs away. Centers for Disease Control and Prevention. http://www.cdc.gov/ounceofprevention/docs/oop_brochure_eng.pdf, Accessed May 13, 2009.
