Even many health care professionals wrongly think everyone carries C. diff in their intestines and that the bug only overgrows when antibiotic therapy or illness disrupts the normal gut ecology and gives it room to grow.

That's not the case. Only 5% of the population is "colonized" by C. diff. And because population studies have only looked at one point in time, even most of these people may only be having a temporary infection.

Even so, more than half of Americans show evidence of a previous C. diff infection some time in their lives. This often happens soon after birth. But infants only rarely get C. diff disease. The reason for this isn't clear, but there's evidence from animal models that C. diff toxins have trouble binding to the immature gut.

C. diff bacteria are very sensitive to oxygen. But C. diff spores are another matter. They are nearly indestructible and can survive for months on dry surfaces. The CDC recommends disinfecting surfaces with bleach, because the usual hospital disinfectants don't affect it.

People with C. diff infection have millions of C. diff spores in their feces. These spores carry the infection to others via what experts indelicately call fecal-oral contact. Careful hand washing rinses the spores from contaminated hands, but alcohol gels won't do the trick.

Two things have to happen for you to get C. diff disease:

• You have to ingest C. diff spores.
• Something has to disturb the ecological balance of the normal bacteria living in your colon.

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