What's the difference between a cold and the flu?
Both the common cold and the flu (influenza) are caused by viruses— but by different types of viruses. The rhinovirus is the most common cause of a cold, while the influenza virus causes the flu. You can usually tell whether you have a cold or the flu based on your symptoms. A fever is rare with a cold, but common with the flu. Headaches, muscle pains, weakness, and fatigue are rare or mild with a cold but more common or severe with the flu. Runny or stuffy nose, sneezing and sore throat are common with a cold but less common with the flu.
Both the common cold and the flu can lead to complications. The common cold can cause sinus congestion and earaches. The flu can lead to more severe problems such as bronchitis and pneumonia. Fortunately, you can prevent the flu and its dangerous complications by getting a yearly flu shot. The prescription medicines oseltamivir (brand name Tamiflu), zanamivir (Relenza), and amantidine (Symmetrel) can help reduce the time it takes to recover from the flu if you start taking them within 48 hours of the start of your symptoms.

Oseltamivir was recently approved for use in preventing the flu. Individuals exposed to the flu from close contacts or those who are at risk for the flu due to community outbreaks can reduce the likelihood of contracting the flu by starting oseltamivir before symptoms arise.

Zanamivir may also prevent flu symptoms. A recent study showed that zanamivir can reduce the likelihood of a getting the flu from an ill family member by 79% when taken within 36 hours of initial symptoms.

Unfortunately, no vaccine or medicines can prevent the common cold or reduce its duration. But over-the-counter medicines such as decongestants, antihistamines and pain relievers can help relieve symptoms of both the cold and the flu.

Sources:
The Merck Manual of Medical Information: Home Edition. Merck & Co., 1997.
Handbook of Nonprescription Drugs. American Pharmaceutical Association, 2000.
Micromedex Healthcare Series. Micromedex Inc., 2000.
Hayden, F. "Inahled Zanamvir for the Prevention of Influenza in Families," The New England Journal of Medicine 2 (2000).
Hayden F. " Use of the Selective Oral Neuraminidase Inhibitor Oseltamivir to Prevent Influenza." The New England Journal of Medicine (28) 1999.
This answer prepared 12/19/00.