Infectious diseases can be caused by:
- Bacteria. These one-cell organisms are responsible for illnesses, such as strep throat, urinary tract infections and tuberculosis.
- Viruses. Even smaller than bacteria, viruses cause a multitude of diseases — ranging from the common cold to AIDS.
- Fungi. Many skin diseases, such as ringworm and 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.
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.
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.
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.
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.
July 23, 2014
- Understanding microbes in sickness and in health. National Institute of Allergy and Infectious Diseases. http://www.niaid.nih.gov/topics/microbes/Pages/default.aspx. Accessed May 20, 2014.
- McPhee SJ, et al., eds. Pathophysiology of Disease. 6th ed. New York, N.Y.: The McGraw-Hill Companies; 2010. http://accessmedicine.mhmedical.com/content.aspx?bookid=339&Sectionid=42811304. Accessed May 20, 2014.
- Levinson W. Review of Medical Microbiology & Immunology. 12th ed. New York, N.Y.: The McGraw-Hill Companies; 2012. http://accessmedicine.mhmedical.com/content.aspx?bookid=400&Sectionid=42098466. Accessed May 20, 2014.
- Longo DL, et al. Harrison's Online. 18th ed. New York, N.Y.: The McGraw-Hill Companies; 2012. http://www.accessmedicine.com/resourceTOC.aspx?resourceID=4. Accessed May 20, 2014.
- Walker BR, et al., eds. Davidson's Principles and Practice of Medicine. 22nd ed. Philadelphia, Pa.: Churchill Livingstone Elsevier; 2014. http://www.clinicalkey.com. Accessed May 22, 2014.
- Stopping the spread of germs at home, work & school. Centers for Disease Control and Prevention. http://www.cdc.gov/flu/protect/stopgerms.htm. Accessed May 22, 2014.
- Basics for Handling Food Safely. U.S. Department of Agriculture. http://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/safe-food-handling/basics-for-handling-food-safely/ct_index. Accessed May 22, 2014.
- Sexually transmitted diseases (STDs). Centers for Disease Control and Prevention. http://www.cdc.gov/std/prevention/default.htm. Accessed May 22, 2014.
- Viral meningitis. Centers for Disease Control and Prevention. http://www.cdc.gov/meningitis/viral.html#transmission. Accessed May 22, 2014.
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