Flue viruses are constantly mutating into stronger and weaker strains. “Random walk” or “flip the coin” models can often replicate the process.
When a very weak strain appears, it tends to spread widely among a population since its victims don’t stay at home in the bed, but rather they circulate widely and infect others.
Conversely, when a very strong strain develops, it tends to spread slowly since its victims are bedridden or even dead before they can infect a lot of their friends and neighbors.
Therefore, the development of lethal flu viruses is normally a self-limiting process.
The deadliness of the 1918 virus is explained in large measure by (1) the conditions in crowded military barracks and hospitals where the normal self-limiting process was nullified and (2) secondary bacterial infections. Such conditions are unlikely to be repeated in today’s USA.
Therefore, even if a highly lethal variety of H1N1 develops in a relatively backward part of Brazil, it’s not likely to sweep the USA or other countries that have good health infrastructures.
Add to the above factors the availability of a vaccine that will be at least partially effective against H1N1, and then I think the probability becomes vanishingly small that a lethal “swine flu” pandemic will come to the USA during the 2009-2010 flu season.
This virus is novel in that it has how many (pig, human, bird) hosts ?
It is not a matter of how lethal it is, it is how many ways this can be transmitted.
The deaths are an indication of the strength and it is getting stronger.
If in a small business half your people are out due to the flu, then it has a great impact on a already broken economy.