QUESTION: What’s the difference between a cold and the flu?

Answer: Both are upper-respiratory-system illnesses caused by viruses, but the flu — short for influenza — is more severe than a cold.

Symptoms of a cold are a sore throat, runny or stuffy nose, and sometimes a headache caused by congestion. If there is a fever, it is mild.

Flu symptoms are sore throat, runny nose, dry cough, headache, extreme tiredness, muscle aches and a high fever.

QUESTION: What causes the flu?Answer: Flu is caused by one of three types of influenza viruses. Influenza type A and influenza type B viruses are the ones that cause serious disease; the third, influenza type C, usually causes only a mild illness and is not considered a public health problem.Because flu is caused by a virus, antibiotics, such as erythromycin or amoxycillin, which are highly effective against bacterial infections, are of no use in fighting the flu.

QUESTION: Is the flu deadly?Answer: It can be for people in high-risk groups, which includes those 65 and older; people with weakened immune systems, such as people with AIDS; people with long-term diseases, such as heart disease or diabetes; and children under 2. Influenza can lead to complications, such as bacterial pneumonia and worsening of chronic conditions such as heart failure.

QUESTION: Why, then, is the flu vaccine offered to healthy people between ages 2 and 65 in most years?Answer: There are two reasons. The first is to prevent sick days and loss of productivity. Companies often offer flu shots to their employees as a way of keeping their work force on the job.The second reason is to create something called “herd immunity.” Flu is a highly contagious disease, but you can’t get it if you haven’t been exposed. And if lots of people have been immunized, there are fewer sick people to whom you could be exposed.

QUESTION: Is there any danger in getting a flu vaccination?Answer: Unwanted side effects from the flu vaccine are extremely rare.

QUESTION: What can people who aren’t going to be getting a shot do to reduce the likelihood of getting the flu?Answer: For healthy people between the ages of 5 and 49 who are not pregnant, the FluMist nasal spray is another way of building immunity to the flu virus. Unlike the injection, which uses a killed virus to stimulate the production of antibodies, FluMist uses a live virus that has been weakened. For this reason, it is not recommended for people with compromised immune systems or any serious disease.Another thing someone can do, particularly an elderly person or a person with a serious chronic disease, is to get vaccinated for pneumococcal pneumonia. Although that won’t help prevent the flu, it can prevent one of the serious complications.

QUESTION: Why has there been so much talk about lines for the flu vaccine?Answer: Because it’s human nature to react to a shortage by hoarding. When the government announced last month that there would be fewer doses of influenza vaccine available than planned, many people suddenly were in a rush to get immunized, including, most likely, some who wouldn’t have gone to the effort to get a shot otherwise.

QUESTION: Why doesn’t the flu vaccine create immunity for many years like others do, such as the measles vaccine?Answer: Unlike the measles virus, the influenza virus is continually changing. It changes in two ways. One is called “antigenic drift,” which is small changes that take place in the virus. Antigenic drift occurs each year. The other is “antigenic shift.” Antigenic shift is a major change, but it doesn’t happen every year. When there is an antigenic shift, the disease will affect many more people, generally causing a pandemic (worldwide flu epidemic).

QUESTION: Why do pandemics so often seem to start in Asia?Answer: The answer isn’t known, but the concentration of people who live in close quarters with other people and in close quarters with animals is the likely reason, said Dr. Arnold Monto, an epidemiologist with the University of Michigan and a leading researcher on influenza.Versions of the influenza type A viruses circulate among birds, pigs, horses and other animals, as well as among humans. An antigenic shift often involves mixing of viruses in an animal host.

QUESTION: What should people do if they get the flu?Answer: Although the penicillin-type antibiotics are of no help, some anti-virals have been created in recent years.One such product is Tamiflu, a pill that is taken for 10 days. To be effective, anti-viral medicines must be started within two days of the onset of symptoms, says Monto.Anti-virals, which are available by prescription only, may also be effective in preventing the development of flu in people who have been exposed to the disease but have not yet come down with it. They have most frequently been used in this way in nursing homes.Other than anti-virals, the way to deal with the flu is to follow mother’s orders: Stay in bed, drink lots of fluids and treat the symptoms with over-the-counter or prescription medications. Just remember not to give aspirin to children who have the flu, because it could cause Reyes syndrome.Sources: www.cdc.gov; “Wall Street Journal” and “New York Times”