The National Academies: What You Need To Know About Infectious Disease

The National Academies

What You Need To Know About Infectious Disease


Schistosomiasis, also called bilharzia, is a disease caused by parasitic worms that live in certain types of freshwater snails. The snails become infected when Schistosoma eggs hatch in freshwater contaminated by urine and feces from infected people and attach themselves inside the snails, where they begin developing and multiplying. At a certain point, the larval form of the parasite leaves the snail and enters the water, where it can survive for about 48 hours. When people come in contact with contaminated water (from swimming, bathing, washing, or wading, for example), they can become infected if the parasites penetrate the skin. During the course of several weeks, the parasites migrate through the person’s tissues and develop into adult worms. Mature worms mate inside the human host and the female produces eggs, some of which travel to the infected person’s bladder or intestine and are passed in urine or stool. Other eggs become trapped in body tissues, causing an immune reaction and progressive damage to organs. The worms are not found in the United States but more than 200 million people are currently infected by this disease worldwide, primarily in tropical and subtropical locations. The disease is second only to malaria as the most devastating parasitic disease.

Symptoms of schistosomiasis are caused by the body’s reaction to the eggs produced by the worms, not by the worms themselves. Eggs from the worms travel to the intestine, liver, or bladder. Those that are not shed during urination or defecation can become lodged in these areas, causing inflammation or scarring. During the early phase of infection, most people do not have any symptoms. For those who do, within days of being infected, a person may develop a rash or itchy skin. One to 2 months after infection, the individual may develop symptoms, including fever, chills, muscle aches, and cough. Children who are repeatedly infected can develop anemia, malnutrition, and learning disabilities. If left untreated, schistosomiasis can persist for years. Symptoms of chronic schistosomiasis include abdominal pain, enlarged liver, blood in the stool or urine, problems passing urine, and increased risk of bladder cancer. Eventually, after years of infection, the parasites can damage the liver, spleen, intestine, lungs, and bladder. In rare cases, eggs can be found in the brain or spinal cord, possibly causing seizures, paralysis, or spinal cord inflammation.

If left untreated, chronic schistosomiasis can result in death. In sub-Saharan Africa, more than 200,000 deaths per year are attributable to this disease. A safe and effective drug called Praziquantel, typically prescribed for 1 to 2 days, is available to treat this disease. But access to the drug is limited in countries where it is most needed.

If you are visiting or living in an area where schistosomiasis is of concern, avoid swimming or wading in freshwater. Ocean water and chlorinated pools are safe. Drink safe sources of water. When bathing, be sure water has been at a roiling boil for at least 1 minute to kill any parasites, then cooled to avoid scalding. Water held in a storage tank for at least 1 to 2 days should be safe for bathing. Vigorous towel drying after brief exposure to contaminated water may prevent the parasite from penetrating the skin, but this method should not be relied on for prevention.


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What do you know about infectious disease?

The human papillomavirus (HPV) causes about what percentage of cervical cancer cases?

  • Sorry, that’s incorrect.

    The human papillomavirus (HPV) causes more than 90% of cervical cancer cases.

  • Sorry, that’s incorrect.

    The human papillomavirus (HPV) causes more than 90% of cervical cancer cases.

  • Correct!

    The human papillomavirus (HPV) causes more than 90% of cervical cancer cases.

Infectious Disease Defined


Short for deoxyribonucleic acid, DNA is any of the nucleic acids that contain the genetic instructions necessary for the development and functioning of all living organisms as well as some viruses.

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