Question: Recently, I had a brief discussion with a coworker about viruses. I said you cannot get the same virus twice. My coworker disagreed. It is my understanding that we build immunity in our bodies due to antibodies formed after every specific viral infection we come in contact with. Am I correct?

Answer: Depending upon the virus and how you were exposed, it is possible to get the same infection more than once. For example, the flu virus can mutate into new strains to which we're not immune - even after we've received a flu vaccine.

Despite getting a shot against a specific strain of flu, the immunity can sometimes be short-lived. That's because the injection is not a live virus but virus protein particles. The body mounts an immunity that is not not necessarily strong. When you get the full-blown flu, there's more time - and a whole lot more virus - for the body to develop a powerful and long-lasting immunity.

What we call a common cold is different. Rather than one virus, it is either a rhinovirus, a coronavirus or an adenovirus. Each has many strains that constantly mutate. So you're getting different strains of a cold, rather than the same one twice.

While you can get chicken pox only once, that virus is never eradicated from your body. It lies dormant within sensory nerve roots in the spinal cord. Under situations of stress or a weakened immune system, the viral particles can rise to the skin's surface to reemerge - as shingles.

Mitchell Hecht is a physician specializing in internal medicine. Send questions to him at: "Ask Dr. H.," Box 767787, Atlanta, Ga. 30076. Due to the large volume of mail received, personal replies are not possible.