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Lower pathogenicity results in the host having a better chance of survival, and thus of transmitting the disease, before the host itself dies of it. With the current high pathogenicity level (over 50% mortality), the host dies too quickly before the disease can be spread. As with the 1918 flu, once it lost some of its pathogenicity (I think overall it had an 8% mortality), it spread faster and easier. If it were to remain at a >50% mortality rate, many hosts would die before transmission could occur, thus the transmission rate would not be nearly as high as if the mortality rate were lower. Viruses mutate, sometimes rapidly, but not evolve. Just as in viruses of past, they can get deadlier or weaker. It's like throwing the dice everytime a host is infected.

No, influenza viruses, all of them, are transmissible BEFORE the host shows any sign of illness. I've heard the reasoning you expound above before, but not from any serious researcher. There is no reason for the virus H5N1 to lose pathogenicity in a supposed tradeoff for easier transmissibility. That kind of statement imputes human reasoning to a semi-living complex molecule. For example, unless the newer drugs intervene, HIV is invariably fatal. No loss of pathogenicity there...

And you may disagree, but mutations are the one and only cause of evolution in my book.

Yes, flu viruses are ALL transmissible before symptoms appear. However, if the host were to die in 3 days from the onset of symptoms, as opposed to two weeks, then he has less chance to infect someone. The longer he lives and interacts with other potential hosts, the more the chance of a transmission. With the current H5N1, symptoms appear on average of 3 days after exposure. So, everyone has a base potential of 3 days to spread the virus. Now, lets say the mortality rate is 100% (highly pathogenic). Well, then death is imminent and the host will be infectious until that time (varies as you know by lots of factors). As we drop the mortality rate, or as the virus loses it's pathogenicity, the host will live longer, as when faced with a weaker virus, his ability to survive it, will increase. Thus, if he is infected with a virus that has a 10% mortality rate, he should be able to stay alive longer, and potentially defeat it, thus increasing the time available to interact with others and infect them. That is what I meant in my statement. I have read/ heard similar propositions from various sources, and that was even discussed in the book I mentioned. Am I way off my rocker here? I'm not a Microbiologist but have read alot about it. If you'd like, message me. I'm always trying to learn more about the potential of H5N1.