Both colds and flu are more common in winter. The Centers for Disease Control and Prevention reports on flu activity from October through May but says the flu season usually peaks between late December and March.

Whenever it hits, a bad cold can be as miserable as a mild-to-medium case of flu, says Edward Janoff, head of the infectious-diseases department at the University of Colorado.

"Both colds and flu can range from no symptoms at all to really sick," he says. Most colds are caused by viruses that attack the upper respiratory system, causing the symptoms we know and hate, such as sneezing, coughing and runny nose. "But the consequences of flu are worse."

Influenza viruses may affect the upper respiratory tract, causing symptoms that come on suddenly, but they also can go deep into the lungs, causing breathing difficulty, pneumonia or bronchitis.

"A bad case of influenza is much worse than a bad cold," he says. "A bad influenza can kill you, and a bad cold won't."

DIFFICULT TO DIAGNOSE

Distinguishing between colds and flu can be tough, even for doctors. A study published in July in The New England Journal of Medicine found only 28 percent of children hospitalized and 17 percent of those treated in clinics who had lab-confirmed flu had been accurately diagnosed by their doctors.

It's not that doctors can't make an accurate diagnosis, says Kathryn Edwards of Vanderbilt University, one of the study's co-authors. "It's not easy to get a rapid diagnosis that is specific and accurate," she says. Rapid tests are available but the researchers found they were not widely used, and "they're only 75 percent to 80 percent accurate."

In the study, paid for by the CDC, lab analyses were used that cost $50 to $100 a test, she says.

"Our first goal was to find out how much flu there was, and we found a lot," she says, an annual hospitalization rate of nearly one child in 1,000.

Making accurate diagnoses even more difficult, she says, is that children may not always have the same flu symptoms as adults.

But quick, correct diagnosis of flu can allow use of antiviral medicines such as Tamiflu or Relenza to shorten the duration of illness, the researchers noted. Tamiflu, available in pill or liquid form, can be given to children as young as 1 year, and Relenza, an inhaled medicine, can be given to children 7 or older. To be effective, both have to be used within two days after symptoms start.

COMMON IN CHILDREN

The take-home message of the study, Edwards says, is that flu in children is more common than realized. "There is lot of flu that causes children to be hospitalized and much more that causes children to see the doctor and get antibiotics," often unnecessarily.

The CDC recently expanded its recommendations for flu vaccine, advising that all children 6 months to 5 years of age should be immunized each year (children need two doses the first year they get flu vaccine, then one a year thereafter).

That's a good step, health experts say, but it might not go far enough for some. Gregory Poland, an infectious-disease specialist at Mayo Clinic in Rochester, Minn., has long advocated a recommendation for annual flu shots for everyone.

A universal flu vaccine recommendation from the CDC would stimulate vaccine makers to produce enough to meet the demand, he says.