Bacteria in your food are a bad thing, usually: Think of E. coli or other harmful bugs.
But there's a whole other world of "friendly" bacteria.
Called "probiotics" and found in yogurt, among other foods, they provide health benefits beyond the regular live cultures found in those foods.
Probiotics are among the fastest-growing category of functional foods, according to the market research firm Mintel, which cites a 140 percent increase last year in the launch of new probiotic-fortified products.
So far, the strongest evidence on probiotics has focused on digestive-tract problems such as lactose intolerance and diarrhea -- including infectious diarrhea among children and the type that develops after a person has taken antibiotics (which wipe out both good and bad bacteria, altering the natural balance of the digestive tract).
Europeans have long embraced the concept of probiotics (which is derived from the Greek, meaning "for life"), but Americans haven't fully warmed up to the idea of downing a drink swimming with billions of live microorganisms.
"We've done a good job in this country of scaring people to death of microbes," said Mary Ellen Sanders, president of the International Scientific Association for Probiotics and Prebiotics, a non-profit scientific organization.
Some of the most exciting research on probiotics involves the potential to boost immunity, said Allan Walker, a professor of nutrition and pediatrics at Harvard Medical School.
Probiotics appear to communicate with the cells in our intestines to turn on antibodies to fight infections, Walker said.
This "cross-talk" helps rev up our immune response so we're better prepared to deal with invading bacteria and viruses that could make us sick, he said.
� A seven-month study of more than 570 children in day-care centers found that intake of a probiotic milk reduced the number and severity of respiratory infections and the need for antibiotics.
If you want a yogurt with probiotics, check the label.