The New Scientist has published this article describing how bugs cause
epidemics, or not as the case may be and the reasons why either occurs. Not
specifically about Bird Flu, but refers to it.

This counters some of the over simplistic posts which have appeared here in
the past.

Sorry I have to post the full thing, the full article is password protected
on the NS Site
http://www.newscientist.com/channel/health/mg19225751.400-the-rules-of-contagion...

FRANCE, 1918: a new, lethal strain of influenza sweeps through the
trenches, killing even young, fit soldiers. Within weeks the outbreak has
spread to ports of call in Africa, North America and onward. Over the next
two years, at least 20 million people worldwide will die in the worst
epidemic since the black death.

DEMOCRATIC REPUBLIC OF CONGO, 2002: Ebola virus resurfaces, killing nearly
90 per cent of those who catch it. The epidemic does not spread much beyond
the country's borders, however, and within months it fades away, leaving a
total death toll of just 128.