In today's society, the hot tub is a popular symbol of relaxation. The steaming water soothes aching muscles, clears congestion, and provides a perfect place to enjoy a drink on a cool autumn evening. But recent reports also suggest that there's a chance -- albeit a very small one -- that those same healing waters can make you sick. It's nothing to panic about, but 24 cases of hypersensitivity pneumonitis (HP) -- a rare respiratory condition characterized by fatigue and other flulike symptoms -- have been traced to hot-tub water.

"Hot-tub lung," as it's been dubbed, is caused by Mycobacterium avium, or MAC. The general theory is that this bug lurks in the aerosol mist that hovers above the water. When you soak in your tub, you're inhaling that mist -- and you could be giving your lungs a healthy dose of unhealthy bacteria. Thus far, cases of the disease have been linked only to indoor tubs, suggesting that insufficient ventilation may play a role. However, Dr. Roy Vore, a microbiologist and member of the National Pool amp; Spa Institute's Recreational Water Committee, says it isn't clear what level of MAC concentration is harmful, so he can't say whether a fan would be an effective precaution.

The disease has been linked to only six unique hot tubs (33 additional cases have been linked to an indoor pool), so various factors must contribute to its transmission. Vore suggests that an individual's body chemistry contributes to his or her vulnerability to HP. "It's kind of like an allergy," he says. "Some people have a particular reaction to MAC."

According to Vore, no one really knows where the bacterium comes from, though "the best bet is it's present in the water used to fill the tub." Even then, it must be present in a certain concentration, and one would have to be exposed to it for a long time, for illness to occur.

It would be a huge overreaction to classify hot tubs as dangerous; the number of "problem" tubs represents an infinitesimal percentage of all units sold. However, Vore says, certain measures should be taken to minimize users' risk. MAC rarely exists on its own, as it thrives in a mixed community of organisms. Therefore, Vore says, "the best recommendation I can give is to use a [hot-tub] sanitizing system registered by the EPA, and to follow the directions at all times." That includes draining the tub on a regular basis.