Outbreak - Anatomy of a potential killer

winnipegsun.com - Outbreak - Anatomy of a potential killer: "Flu viruses undergo mild changes every year, which creates a need for annual flu shots to combat each new strain.
About three times each century, a dramatic shift in an influenza strain occurs, creating a new version the human immune system has no antibodies or immune cells to fight.
Pandemic influenza begins when that new strain causes significant illness and spreads efficiently from human to human around the world.
Whether it's a deadly strain of H5N1 avian influenza or begins from a different Influenza A flu strain, a pandemic may still be years away.
The incubation period would be about one to three days. "