What causes avian flu, in humans, to be so deadly?
The answer lies in a detailed understanding of the genetics of the particular deadly strain and how it affects humans. Not all the details are known but the type of hemagglutinin (HA) is important. The avian influenza hemagglutinin (H5) binds to a receptor called "alpha 2-3 sialic acid receptor" but the normal human influenza HA (H1) binds "alpha 2-6 sialic acid receptors". Human lungs have alpha 2-6 sialic acid receptors near the top of the lungs and alpha 2-3 sialic acid receptors mostly near the bottom of the lungs. That means, when a person is infected with a virus carrying the H1 version, the infection tends to be limited to the upper respiratory tract. That makes it easy to spread but also easy for the body to battle it. Coughing brings up goo from the upper respiratory tract more easily than from the lower respiratory tract, so it is easier to recover from infection by H1. On the other hand, H5 binds to the lower respiratory, making it harder to shift (cough
