I am not a doctor of course but my understanding is the flu can be any virus that makes you ill with flu like symptoms, nausea, diarreah, aches, pains etc. It is a catch all phrase for many different virus strains. There are all kinds of virus that cause a myriad of symptoms and they mutate too and change. Some obviously are worse than others. Flu shots immunize against the most commons strains that are currently around but they cannot cover all strains or if the virus mutates then the immunization may not be effective. Wish a doctor here would pipe in and clarify.

On a recent cruise I was sailing with two doctors. One strongly thought the flu shot was not worth the risk, the other thought it was definitely worth the risk particularly for those "at risk" because they were older or had other health problems. Varying opinions!