It is a vaccination. Flu is influenza, an illness caused by a virus. There are many types of flu which are called strains. These strains have names like Hong Kong flu, influenza A, etc. Each year the strains of flu that people can catch are a little different.
Flu is a respiratory illness.
It gives you fever and chills, cough, runny nose, and muscle aches. Flu can also cause pneumonias which could lead to hospitalization or death. Flu season is in the winter generally November to March or April. It usually starts earlier in areas that have early cold weather. People who should get a flu shot are those who are most likely to get very sick with the flu.