The common cold is caused by any of a number of viruses that can involve the upper respiratory tract. As your body reacts against the attacking virus, you get the symptoms of a cold -- including nasal congestion, sneezing, and sniffles.

What causes the flu?
What's the difference between respiratory flu and intestinal flu?

Influenza, or "the flu," is caused by one of several strains of the influenza virus. These viruses usually cause symptoms in the head and chest -- such as congestion, sore throat, or cough -- but also can cause intestinal symptoms such as nausea and vomiting. The influenza virus also can have general symptoms such as fever, muscle aches, chills, or sweats.

Are people with strong lungs -- runners or dancers, for instance -- less susceptible to a cold or the flu?

Regular exercise, from aerobics to walking, boosts the immune system. People who exercise may still catch a virus that causes a cold or the flu, but they may have less severe symptoms and may recover more quickly than their friends who do not exercise.

Are people who have asthma or allergies at greater risk for catching a cold or flu?

Yes. People who have asthma or allergies may already have inflamed and irritated lungs. Their bodies are already fighting one irritant or illness, so it is often more difficult for them to fight off a virus. Smokers are also more likely to have irritated lungs and are thus more likely to catch a cold or the flu.

Are you contagious before you show symptoms? If a co-worker is sniffing over the phone you share, is he or she contagious then?

The viruses that cause colds and the flu are spread by microscopic droplets from an infected person's respiratory system. It may be possible to catch the virus before symptoms show, via close contact such as kissing or sharing eating utensils. However, people generally need to have some symptoms, such as a runny nose or cough, in order to be contagious. When your co-worker shows symptoms, he or she may indeed be spreading contagious droplets to your shared phone, to the doorknob of your office, or to the buttons on the fax machine. Your best defense is to wash your hands frequently.