A deadly bacterial illness commonly seen in people taking antibiotics appears to be growing more common, even in patients who are not taking such drugs, federal health officials warned on Thursday.

The bacterium, Clostridium difficile, has become a menace in hospitals and nursing homes, and last year it was blamed for 100 deaths over 18 months at a hospital in Quebec.

Recent cases in four states show that the bacteria are appearing more often in healthy people who have not been admitted to hospitals or even taken antibiotics, according to the Centers for Disease Control and Prevention.

"What exactly has made C-diff act up right now, we don't know," said Dr. L. Clifford McDonald, an epidemiologist at the centers.

C. difficile is found in the colon and can cause diarrhea and a more serious intestinal condition, colitis. It is spread by spores in feces that are difficult to kill with conventional household cleaners.

The bacteria have grown resistant to certain antibiotics that work against other colon bacteria. A result is that when patients take those antibiotics, particularly clindamycin, competing bacteria die off and C. difficile multiplies exponentially.