DEAR DR. ROACH: What is cellulitis, and how does one get it? I live in a retirement facility, and two ladies at my dining table have it. It seems to be very painful and difficult to heal. Should I take any precautions? — R.G.

ANSWER: Cellulitis is an infection of the skin. It involves the full thickness of the skin, so it differs from the related infection erysipelas, which is an infection of just the outer layer.

Most cases of cellulitis are caused by Streptococci, but Staphylococcus aureus (“staph infections”) are increasingly problematic, especially those caused by methicillin-resistant Staphylococcus aureus, also called MRSA.

The biggest risk factor for developing cellulitis is a lack of skin integrity. Trauma to the skin, such as a simple abrasion, a small cut or an insect bite, is a common way for the bacteria, which normally live on (“colonize”) the skin, to get through the skin barrier and cause an infection. Skin conditions such as eczema, or an infection with, say, a fungus are other ways the bacteria can get in. Chronic edema — from heart, liver or kidney disease, from lymphedema or from medications — can cause small cracks in the skin that can’t be seen easily but that are large enough to allow bacteria to enter.

Cellulitis can occur anywhere on the body, but it is most commonly seen in the legs and feet. Having a weakened immune system — from any cause, including disease or treatments that affect the immune system — predisposes one to developing cellulitis. Longstanding diabetes, which affects small blood vessels, is another risk factor. Just plain dry skin can cause subtle cracks. As the immune system tends to wane with age, advanced age itself is a risk.

Person-to-person transmission of cellulitis is very rare, since it is predominantly something about the person, not simply exposure to the bacteria, which is most important in developing cellulitis. Proper skin care is the key for people at high risk, which includes anyone with a history of cellulitis as well as anyone with the risk factors above. This means regular application of moisturizers in people with dry skin, careful nail care, good footwear to protect the feet from trauma and prompt care of any skin condition.

DEAR DR. ROACH: In a recent article, you answered a woman in her 80s whose physician was no longer ordering mammograms or colonoscopies for her, and your response mentioned a possible risk of damage from colonoscopy. Can you clarify what you meant? All we hear is how important they are and how painless the procedure is, etc. — C.S.L.

ANSWER: All medical procedures have risk. The risk of a serious complication from a colonoscopy is low, about 25 in 10,000 procedures; that risk probably is higher with advancing age. Even though 99.75 percent of people won’t have a serious complication from a colonoscopy, I still think it’s important to recognize that it has risks. However, for most people between 50 and 75, the benefits greatly outweigh the risks.

One reader wrote in to ask me about CT colonography, sometimes called a “virtual colonoscopy.” I think this technique may replace colonoscopy for some people. Perhaps eventually people will have a CT scan and, if an abnormality is found, will go for a colonoscopy the same day to evaluate the abnormality. I don’t think the technology is there yet to recommend this strategy, though.

READERS: The booklet on herpes and genital warts explains these two common infections in detail. Readers can obtain a copy by writing: Dr. Roach — No. 1202, Box 536475, Orlando, FL 32853-6475. Enclose a check or money order (no cash) for $4.75 U.S./$6 Can. with the recipient’s printed name and address. Please allow 4-6 weeks for delivery.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Health newsletters may be ordered from www.rbmamall.com.