#Nice speculation about the functional reason:#
franklin at ug.cs.dal.ca (Steve Franklin) writes:
>The question then would be - how come this super-strain doesn't spread all
>over and nuke us all in a giant plague? Well, it would seem possible to me
>that as the nature of the bacteria changes, so too would it's means of
>transmission. If the bacteria prefers to get into your bloodstream and
>tissue, rather than into your throat and lungs, then transmission of the
>bacteria becomes more difficult - they lose their airborne advantage, and
>require transmission via open wounds.
If the pathogen can only reach its goal (survival, reproduction,
spreading> by killing its host, selective pressure to become more
virulent will be on it. It may (mustn't> than eventually extict itself
with a hyper-sensitive host. But if it can spread from one living host
to the next, coevolution may lead to a more avirulent pathogen and a
more tolerant host. There is one example, where the avirulent
vaccine-strain has spread on the cost of the virulent form, but I
don't know the english name for it <`Pocken' in german>.
#Here comes a speculation about the historical reason:#
Assuming this type of coevolution happened to streptococcal bacteria,
the virulent form might be a kind of atavism, or the relict of the
P.S.: I want to thank, though I did it already via e-mail, Peter W.
Pappas for his answer, officially.