Once thought to be a harmless yeast, blastocystis is a parasite, a microscopic single-celled organism (protozoan). Many protozoans inhabit your gastrointestinal tract and are harmless or even helpful; others cause disease.

Whether blastocystis is the type of protozoa that causes disease is controversial. While many people who carry blastocystis have no signs or symptoms, the organism is also found in others who have diarrhea and other gastrointestinal problems. Because blastocystis often appears with other organisms, experts aren't sure whether it causes disease on its own or is an innocent bystander in this setting.

It's also possible that some people may be carriers of blastocystis and don't exhibit any signs or symptoms of infection, while other people are more susceptible to infection.

Many types of protozoans get into the intestinal tract through oral-fecal contact, such as occurs when a person doesn't wash his or her hands thoroughly after using the toilet before preparing food. No one knows for certain how blastocystis is transmitted, but experts suspect it's through oral-fecal contact. Experts do know that the prevalence of blastocystis increases in places with inadequate sanitation and poor personal hygiene.

Jan. 25, 2013