From gallstones to arthritis to heart disease, many illnesses long presumed to have roots in genes or lifestyle may be caused largely by infectious agents, a growing number of scientists believe. Most of the evidence remains circumstantial. A microbe may be suspiciously present in people who have a disease, for example, and not in those who don't, suggesting causality.

For some conditions, many scientists feel certain that microbes play at least a contributing role where none was suspected previously. The implications of this new theory are enormous, researchers say. Most important, it suggests that vaccines or antibiotics may have an unexpectedly big role to play in the treatment of chronic diseases that today are treated with only modest success through lifestyle changes, such as exercise and improved diet.

"If an infectious agent is responsible for even a portion of these diseases, that could change the outlook for treatment and prevention dramatically," said Barry Bloom, dean of the Harvard School of Public Health.