Does the strain of flu that gets the majority of people sick every year stay the same or change?
If it changes, how does the vaccination work? Do they know what strain will be going around and vaccinate for that strain?
Influenza A viruses are the "same" the world over, but certain markers on the surface change season to season-which prevents the development of lasting, permanent immunity, such as to mumps or measles.
The CDC tries to predict which strains will be active in the northern hemisphere by looking at what was most prevalent in the preceding winter in the southern hemisphere; sometimes they're spot on [which appears to be the case this year] and sometimes not even close. A couple years ago, they completely missed the mark and the shot offered no protection.
There is some evidence that getting a flu shot every year leads to the development of "pooled immunity" to influenza strains, giving one some lasting protection.
Influenza A is a predominently respiratory infection, if you've got much in the way of GI symptoms, you probably don't have it. If you're in bed for a week, think you're going to die, and wish you could, that's probably influenza.
