Ducks play a big role

http://news.yahoo.com/s/ap/20080324/ap_on_he_me/bird_flu_rice_fields

so far so good

Natural News reports that one of the primary factors
that keeps bird flu from infecting humans is that
the virus has evolved to reproduce most effectively in the bodies of birds,
which have an average body temperature of 106°F (41°C).
 Humans, in contrast, have an average body temperature of 98.6° (37°C),
with temperatures in the nose and throat even lower (91.4°, or 33°C).
This vast temperature difference makes it very difficult for the bird flu
virus to survive and grow in the human body.