Flu viruses travel through the air in droplets when someone with the infection coughs, sneezes or talks. You can inhale the droplets directly, or you can pick up the germs from an object — such as a telephone or computer keyboard — and then transfer them to your eyes, nose or mouth.

People with the virus are likely contagious from the day or so before symptoms first appear until about five days after symptoms begin, though sometimes people are contagious for as long as 10 days after symptoms appear. Children and people with weakened immune systems may be contagious for a slightly longer time.

Influenza viruses are constantly changing, with new strains appearing regularly. If you've had influenza in the past, your body has already made antibodies to fight that particular strain of the virus. If future influenza viruses are similar to those you've encountered before, either by having the disease or by vaccination, those antibodies may prevent infection or lessen its severity.

But antibodies against flu viruses you've encountered in the past can't protect you from new influenza subtypes that can be very different immunologically from what you had before.

Aug. 26, 2015