Is it a cold or the flu?

"The first 48 hours were the worst. My back ached so bad I thought it would break. One minute I was freezing, then I was hot and drenched in sweat. After my fever broke, I developed a hacking cough. And then the weakness—as if a steamroller ran me over. It took me two weeks to get back to work half-time."

That description of typical flu symptoms was relayed to me by a 25-year-old former college athlete. But flu symptoms aren't always typical, and they often blend in with those of other respiratory infections, including the common cold.

Remember the last time you came down with what you thought was the flu? Tired, achy, stuffed nose, sore throat—the works. Well, it probably wasn't the flu. Genuine influenza usually doesn't cause much nasal congestion or a sore throat, but the common cold almost always does. And the distinctions don't end there.

There's no harm in calling a cold the flu. But it can be deadly to assume that a case of influenza is just a bad cold, especially if you're over 65 or for some other reason at high risk for serious complications. Because we're now in the midst of flu season, which usually runs from December to as late as April or May, it's important to know how to tell the difference between the two ailments.

Two of flu's main symptoms are extreme fatigue and weakness. But older people and those with a chronic illness might already have such complaints. In that case, it's important to watch for sudden worsening.

In fact, sudden is the key word. The most characteristic sign of flu is its abrupt onset. A cold can build over a few days, but the flu strikes hard and fast—fever, chills, severe muscle aches, a hacking cough, and general malaise. Fever is a good yardstick, because a cold rarely raises body temperature more than a degree or so.

In some cases, influenza can progress to life-threatening pneumonia. Older people and those with heart or lung problems or diabetes are most vulnerable and should consult a doctor as soon as flu symptoms strike. Younger, healthy people can generally wait a few days to see if flu symptoms persist.

With nothing more than bed rest and plenty of fluids, most normally healthy people recover from the flu within a week or two, though fatigue and weakness can linger for weeks.