Originally Posted by newB

What?

Viruses do not live off dead bodies. Viruses cannot replicate on their own and instead make use of a living hosts replication/division mechanisms.

Also, viruses are highly mutable, so the more people who catch the flu results in even more variants - thus reducing the efficacy of any particular vaccine. Each year, the flu vaccine is a 'best guess' as to which strains are thought to pose the greatest risk.