Over the last several decades, numerous research studies have looked into that claim. Findings from those studies have shown that probiotics do, in fact, appear to improve the body's immune response. That in turn helps the body be better prepared to fight off certain infections, including the common cold.

That doesn't mean if you eat yogurt you will never have a cold. But taking probiotics regularly can lower your chances of getting a cold. And, if you do catch a cold, probiotics may help reduce the severity of your symptoms and shorten how long they last.

As for vitamin C, findings from research studies show that taking 200 milligrams regularly does not help prevent a cold. But it may help decrease the severity and duration of symptoms. Studies also show that vitamin C may be better at decreasing symptoms when taken regularly, rather than just when the cold symptoms start. Vitamin C was more effective in preventing a cold from happening only in people under extreme physical stress or in cold temperature conditions, such as in marathon runners and soldiers in subarctic temperatures.

For adults, the recommended amount of vitamin C is 65 to 90 milligrams a day, and the upper limit is 2,000 milligrams a day. For most people, a large orange, or one cup of sliced strawberries, chopped red pepper or broccoli provides enough vitamin C for the day.

If you take a vitamin C supplement, stick to one that provides about 100 percent of the amount you need in a day. Avoid "megadoses" that have, for example, 500 percent of the amount you need daily. When it comes to vitamin C, more is not always better. Too much may cause diarrhea, nausea, abdominal bloating and cramping, heartburn and headaches.

Of course, there are plenty of other steps you can take to help keep from catching a cold that don't involve taking anything. Wash your hands regularly, keep kitchen and bathroom surfaces clean, and don't share drinking glasses or utensils with others. Using alcohol hand sanitizers is an excellent way to keep hands clean while on the move.

If you do get a cold, you can help prevent it from spreading to those around you by coughing or sneezing into a tissue or the bend of your elbow, rather than your hands. The old recommendations to drink lots of fluids and get plenty of rest won't cure your cold, but they can help you feel better. Having a bowl of chicken soup is a good idea, too.

— Abinash Virk, M.D., Infectious Diseases, Mayo Clinic, Rochester, Minn.