after I posted little info to a discussion group re H5N1 and conservation, this message from a virologist:

Just a little point about influenza in humans- transmission is largely before any illness, as the peak of viral shedding occurrs before interferon release and the specific immune response e.g. T-cell response. This curtails viral replication and reduces shedding.

Any person admitted to hospital ill from influenza will have already transmitted the virus to another/s. The epidemic peak is very sharp in human influenza and it is probably the percentage of immune individuals in the population that brings the epidemic to an end. Therefore I don’t agree with your evolutionary biology idea.

It may be that the number of immune persons in the population after the circulation of the virus for a year forced changes in the HA molecule to escape from neutralising antibody and this had an effect on the virulence of H1N1 but the reason for the virulence of that virus and why it arose and then changed is I believe not known.

I sent a reply:

– evolutionary biology not my idea!

Is peak of shedding always before main symptoms? I know little of this, but some info from WHO suggests virus shedding peaks with symptoms.
How do asymptomatic people transmit virus – just by talking (if not coughing, sneezing)? – and if lower probability of transmission this way, might this have an impact on virus evolution? – to evolve/sustain a virulent flu, maybe need fair percentage of those infected to be able to transmit to others?
Sadly, I’ve seen only fairly brief info from Paul Ewald, not his book, Evolution of Infectious Diseases.

also contacted Wendy Orent, who responded:

I believe she is incorrect about transmission before any illness. There may be (in human influenza after it is fully adapted to the human species) some slight transmission before symptoms set in. But not much. It would have to be shed by breathing and talking – these are not efficient means of transmission. What are symptoms for? Why do we ccough and sneeze?/ Viruses settle in the upper airways and irritate us precisely in order to get us to cough and sneeze. In certain diseases, measles for instance, you transmit fairly early in the course of the disease, before you’re bed-ridden. Measles makes you sneeze. But you are still symptomatic!

Of course, before the disease has adapted to human beings, transmission tends to happen late in the illness, e.g. SARS. Had SARS continued to transmit, it would have adapted to people by becoming a more efficient shedder and spreading earlier in the course of the infection – it would have evolved to mildness like all coronaviruses, which are just common colds in people. It didn’t have that chance – it was wiped out before it became efficiently transmissible.

Just read your answer [after I’d sent in second email], and you are absolutely right – except for the bit about the “fair percentage” – I think you are still thinking in population terms.

She is also, I believe, incorrect about the “not known.” We have a very good idea why the virulence evolved, and why it diminished over time.

[As noted above hin this thread I believe, but I sent to discussion group, maybe useful as summary:] I learned of evolutionary biology and diseases thro Wendy; don’t know all about it by any means (must read Ewald’s book!) – till then believed a monster human h5n1 pandemic flu was imminent. But to me, seems good, and explains a few things re flu that I’d otherwise find puzzling:
– 1918 flu occurring at same time as major world war (with trench warfare)
– human flus otherwise normally of low virulence
– wild avian flus mild (maybe even 1961 in S Africa common terns was from farms)
– ready transformation of wild flus in poultry farms, to viruses that are highly pathogenic for poultry (and, now, wild birds)
– inability of wild birds to sustain HPAIs (not just H5N1)

To me interesting that when Wendy mentioned to Paul Ewald re some ducks being able to survive H5N1, he predicted they shed only low amounts, as observed.