The end result is a virus with the exterior proteins of the H1N1 swine flu -- so the immune system recognizes it -- but with the inner mechanics -- the fast-growth ability -- of X-157.

CNN: Candidate virus for H1N1 vaccine arrives at CDC

Some doctors paint a scary worst-case scenario: A flu pandemic breaks out and beats down the immune systems of millions of people, and then staphylococci--the hospital strain and the nastier variant in the community--run wild on a killing spree.

FORBES: Germ Warfare

Finally, the flu shot does not always work -- particularly among the elderly and people with weak immune systems.

CNN: Flu shot myths addressed