What made the 1918 flu contagious?

Pandemic flu may be only two mutations away - health - 01 February 2007 - New Scientist Space

What a virus needs to spread, the CDC team concluded, is an ability to bind 2,6 sugars, whether or not it needs this to replicate... One clue, they speculate, is that ferrets with non-contagious viruses... do not sneeze. Contagious ferrets do.

“The cells with 2,3-sialic acid receptors have been associated with the bronchial mucins,” Tumpey told New Scientist. This viscous secretion might inhibit these viruses, and prevent the irritation that causes sneezing, which “may contribute to the spread of influenza, at least in ferrets”.

...the CDC results suggest finding out what mutations make H5N1 bind to 2,6-sialic, as those could make it contagious.