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Causes
By Mayo Clinic staffFlu 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.
The flu is caused by three types (strains) of viruses — influenza A, B and C. Type A can be responsible for the deadly influenza pandemics (worldwide epidemics) that strike every 10 to 40 years. Type B can lead to smaller, more localized outbreaks. And either types A or B can cause the flu that circulates almost every winter. Type C has never been connected with a large epidemic.
Type C is a fairly stable virus, but types A and B are constantly changing, with new strains appearing regularly. Once you've had the flu, you develop antibodies to the strain that caused it, but those antibodies won't protect you from new strains. That's why doctors recommend getting a flu shot every year.
