Giardiasis, an illness that affects the digestive tract,  is caused by a microscopic parasite called Giardia lamblia. The  parasite attaches itself to  the lining of the small intestines  in humans, where  it sabotages  the body's absorption of fats  and carbohydrates from digested foods. 

Giardia is  one of the chief  causes of diarrhea  in the United States,  and is transmitted through contaminated water. It can survive  the normal amounts of chlorine used to purify community water supplies,  and can live for more than 2 months  in cold water. As few as 10 of  the microscopic parasites in a glass  of water can cause a severe case  of giardiasis  in a human being who drinks it. 

Young kids are three times more likely to  have giardiasis than adults, which leads some experts to believe  that our bodies gradually develop some form of immunity to the parasite as we grow older. But it isn't unusual for an entire family to have giardiasis,  with some family members having diarrhea, some just crampy abdominal pains, and others with few  or no symptoms. 

Signs and Symptoms 

It's estimated  that between 1% and 20% of  the U.S. population has giardiasis,  and this figure may be 20% or higher in developing countries, where giardiasis  is a major  cause of epidemic childhood diarrhea. But more than two thirds of people who  are infected may have no signs or symptoms of illness, even though  the parasite is living in their intestines. 

When  the parasite does cause symptoms,  the illness usually begins with severe watery diarrhea, without blood or mucus. Giardiasis affects  the body's ability to absorb  fats from the diet, so  the diarrhea contains unabsorbed fats. That means that the diarrhea floats, is shiny, and smells very bad. 

Other symptoms include: 

• abdominal cramps 
• large amounts  of intestinal gas 
• an enlarged belly from  the gas 
• loss  of appetite 
• nausea and vomiting 
• sometimes a low-grade fever 

These symptoms may last for 5 to 7 days or longer. If they last longer, a child may lose weight  or show other signs of poor nutrition. 

Sometimes, after acute (or short-term) symptoms of giardiasis pass, the disease begins a chronic (or more prolonged) phase.