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nce you've been infected with a particular strain of a virus, you develop antibodies that make the likelihood of re-infection very low. Even if the virus were still hanging out on your toothbrush after you recovered—colds and flus can survive there in an infective state for anywhere from a few hours to three days—those antibodies should keep you from contracting the same illness twice. Your toothbrush is no more dangerous while you're still sick, since the viral load on the bristles is negligible compared with what's already in your system.

It is possible to re-infect yourself with bacteria, however. If you were afflicted with strep throat, for example, a colony of streptococcal bacteria might end up on your toothbrush and remain there long enough to give you a second case after you'd taken a course of penicillin. But that threat might be mitigated by toothpaste, which sometimes contains antibacterial compounds.

It is possible to catch a cold, a bacterial infection, or even a blood-borne disease such as Hepatitis B or C from someone else's toothbrush.* (It's an especially bad idea to use a sick person's toothbrush while the bristles are still wet.) Even if you don't put it in your mouth, the infected implement might contaminate another toothbrush nearby: When two are stored in the same cup, their bristles sometimes come into contact. A dirty toothbrush might also pass bacteria or virus particles to the rim of a toothpaste tube, and then on to another toothbrush from there. Another questionable practice: storing your toothbrush so close to the toilet that spray from the flush can reach its bristles, especially in a shared bathroom. As this episode of Mythbusters points out, the presence of some fecal coliforms on your toothbrush won't necessarily make you sick, but the spray from toilet water has been known to spread noroviruses, which are responsible for outbreaks of gastrointestinal illness on cruise ships and in other places.