Identify the enemy

Flu is like the cold in many ways-- basically, they're both respiratory infections caused by viruses. If a cold is misdiagnosed as flu, there's no problem. At worst, a cold can occasionally lead to secondary bacterial infections of the middle ear or sinuses, which can be treated with antibiotics. But if the flu is misdiagnosed as a bad cold, potentially life-threatening flu complications like pneumonia may be overlooked.

Some of the symptoms of a cold and flu are similar, but the two diseases can usually be distinguished. ( See chart below. )

Is It a Cold or the Flu?

Typically, colds begin slowly, two to three days after infection with the virus. The first symptoms are usually a scratchy, sore throat, followed by sneezing and a runny nose. Temperature is usually normal or only slightly elevated. A mild cough can develop several days later.

Symptoms tend to be worse in infants and young children, who sometimes run temperatures of up to 102 degrees Fahrenheit (39 degrees Celsius). Cold symptoms usually last from two days to a week.

Signs of the flu include sudden onset with a headache, dry cough, and chills. The symptoms quickly become more severe than those of a cold. The flu sufferer often experiences a "knocked-off-your-feet" feeling, with muscle aches in the back and legs. Fever of up to 104 degrees Fahrenheit (40 degrees Celsius) is common. The fever typically begins to subside on the second or third day, and then respiratory symptoms like nasal congestion and sore throat appear. Fatigue and weakness may continue for days or even weeks.

"The lethargy, achiness and fever are side effects of the body doing its job of trying to fight off the infection," according to Dominick Iacuzio, Ph.D., influenza program officer with the National Institutes of Health (NIH).

Influenza rarely causes stomach upset. What is popularly called "stomach flu"--with symptoms like nausea, diarrhea and vomiting--is technically another malady: gastroenteritis.

Cold and flu-like symptoms can sometimes mimic more serious illnesses like strep throat, measles, and chickenpox. Allergies, too, can resemble colds with their runny noses, sneezing, and general miserable feeling.

If symptoms persist, become severe or localized in the throat, stomach or lungs, or if other symptoms such as vomiting and behavioral changes occur, consult your physician. "With the typical symptoms, it's not necessary to contact your physician immediately," Iacuzio says.