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The biggest vulnerability of hospital patients is that their Social Security numbers often double as a medical identifier. For identity thieves, "Social Security numbers are the key to the golden kingdom," says Mari Frank, a California attorney specializing in identity theft.

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Often, the culprit in medical settings is a rogue employee. Identity-theft experts recommend that patients and loved ones protest any visible use of Social Security numbers, such as on wristbands or unguarded charts. At the very least, patients may be able to darken a couple of numbers. Patients should refuse to answer aloud any verbal request for those numbers when they might be overheard.

Patients should also resist the impulse to trust their fellow patients. "If you and the other guy were at the counter at Costco, you'd be careful in a way that you're not when you're wearing hospital gowns," says Mr. Cox, the Michigan attorney general. His office recently extracted a guilty plea from a cancer patient who stole the identities of nine other cancer patients. The best thing to do is, if at all possible, have the hospital assign you a non-Social Security number for identification purposes.