Illnesses caused by organisms found in water may have symptoms that are similar to food poisoning. These illnesses include:

• The cryptosporidium parasite, which is often spread by contaminated water or hands. The use of manure to fertilize salad vegetables also is a possible source of infection. Incidence is highest in day care centers and institutional settings; thorough hand-washing usually prevents its spread. Symptoms include abdominal pain, nausea, vomiting, and watery diarrhea. These symptoms usually last 2 to 4 days in a healthy person. A slight fever occurs about half the time. People with AIDS whose immune systems are weakened and are unable to fight infection may have symptoms of cryptosporidium infection for life.
• Gastrointestinal viruses, which cause mild symptoms that last from 24 to 60 hours. Symptoms may include nausea, vomiting, abdominal pain, and diarrhea. Headache and low-grade fever may also occur. Symptoms usually appear 2 to 4 days after exposure to the virus. These viruses are usually spread from person to person, although they also can be spread by eating shellfish grown in contaminated waters.
• The parasite Giardia lamblia, which causes giardiasis (also called giardia). The symptoms of giardia include diarrhea, gas (flatulence), abdominal cramps, nausea, and a general unwell feeling (malaise). Giardiasis is typically found where water treatment facilities are limited or where general sanitation is poor. In the United States, Giardia lamblia is most often found in streams, rivers, and lakes (surface water sources) in the mountains of the West. For more information, see the topic Giardiasis.