For example, people get roundworms, which can lay up to 200,000 eggs a day in the human gut; or whip worms that can live several years in the human body.  And the symptoms these worms cause vary a lot.  People infested with worms may have just some vague abdominal pain, perhaps some nausea or diarrhea.  Roundworms can also cause a blood tinged sputum, and in children they've been responsible for severe intestinal obstruction, pain and problems sleeping.  Pin worms are notorious for the insufferable itching they cause around the anus, especially at night.  Other types of worms can cause bloody diarrhea, liver abscesses, and lung irritation.

Hookworms are famed for causing anemia, and are said to be the most common cause of iron deficiency anemia in the world.  But perhaps the most common symptom of worms -- is none at all.  Some people harbor intestinal worms for years even decades with few obvious symptoms.

There are some ways to avoid intestinal parasites.  Wearing shoes outside can help up avoid roundworms; thoroughly cooking fish, beef and pork can kill tapeworms.  Finally, clean water and good sanitation can help keep the lid on worms generally, and prevent them from exploiting us.  For a copy of this script access our web site, speakingofhealth.com.  Speaking of Health, I'm Dr. Steven Andrew Davis for CBS News.