Q: How do you tell whether you have the flu or a cold?

A: If you have the flu, you’ll know the difference. It will put you on your back for a good spell.

The flu has a sudden onset with symptoms that include high fever and chills, cough, runny or congested nose, sore throat, intense fatigue, severe muscle aches and headache.

The common cold is much less severe, with symptoms such as low-grade fever, cough, runny or congested nose, sneezing and mild sore throat.

Q: You said that the flu shot can’t give you the flu, but my sister came down with the flu shortly after receiving the shot. How can this be?

A: The flu shot vaccine contains killed viruses, so cannot cause the flu. The most likely explanation is that your sister was exposed to the flu virus shortly before getting the shot or during the “lag time” after the shot. Lag time means that it takes about two weeks from the time you get the shot for your body to build up adequate antibodies in response to the vaccine. During this interim period you can still get the flu.

However, the flu vaccine is not infallible. Each year the CDC tracks flu virus strains worldwide in an attempt to predict which new strains will be circulating during the coming flu season. Based on this information, the agency recommends the makeup of that year’s flu vaccine.

When virus strains in the vaccine are well-matched to actual virus strains circulating about, the flu vaccine has been determined to be 70-90 percent effective in preventing the flu among healthy adults under age 65. Not perfect, but pretty good.

Researchers are busy sequencing the genes in the human influenza virus.

This information will enable scientists to quickly zero in on currently circulating virus strains and their variants, which should lead to more effective vaccines.

Q: How is the flu virus spread?

A: The virus is transmitted via air droplets (from coughing) and contaminated surfaces (anything people have touched) such as door knobs, self-service gas nozzles, and currency.

People commonly become infected by touching their eyes, nose, or mouth after their hands have picked up the flu virus.

The best way to avoid the flu, besides getting vaccinated, is to wash your hands frequently. The importance of handwashing can’t be overemphasized. It also prevents the spread of other communicable diseases. I cringe a bit when I read that effective handwashing requires using soap. It’s true that’s the ideal way. But some people, seeing this instruction, are likely to just forego handwashing whenever soap is unavailable.

That’s a mistake. Plain water and scrubbing will rid your hands of most contaminants.

You can also stall the spread of the flu by covering your mouth with a disposable tissue when you cough or sneeze.

Those who remain flu-free won’t be sick and won’t be part of the chain spreading the disease to others. Please do your part this flu season.