See "Death and the Outcast" , New Scientist 26 Feb 2005 p52
The body of a collegue of Caiaphas ( of Biblical notoriety ) was found to
have DNA of both Mycobacterium Tuberculosis and M. Leprae . Further research
of the DNA of bodies stretching from 4th Century AD till the late Middle
Ages confirmed the co-infection .
TB did not protect against leprosy . Leprosy caused a cell-mediated defect
in the immune system . TB then killed off the leprosy carriers quickly .
Hence the demise of leprosy .
It seems that diseases that degrade the immune system (like HIV , Leprosy)
are in inherent evolutionary conflict with ordinary diseases . The
evolutionary pressure on both is to conserve the host's body (the Commons) .
Ie to slow the damage and eventually reach an accommodation allowing the
survival of both .
But if degradation of the host's immune system has taken hold , this
Tragedy of the Commons : for the various organisms not involved with the
immune suppression (bacteria ,etc) , the optimal strategy then is
maximization of replication and denial to competitors . Lethality genes and
replication genes are switched on .
The arms race is then between the HIV to extend it's infectiousness and the
large number of fellow-traveller organisms turned feral to get as much
resources as quickly as possible .
This evolves to a paired system :
1. The immune suppressants try to extend the infectious period. In effect to
prolong the hosts life .
2. The opportunistic diseases evolve to switch on lethality as soon as
infection by the immune suppressant is recognised . In effect , to kill the
host as soon as possible .
The HIV becomes milder (ie leprosy) , while the most successful
opportunistic diseases becomes more virulent .
At some stage the HIV infected organisms are killed off quicker than the
infection rate and the epidemic peters out .
This is repeated over and over again throughout history .
Sadly , it thus seems likely that the present AIDS epidemic arose mainly
because of anti-biotics . The success of anti-TB , measles , polio etc
vaccination campaigns contributed .
A Gaian feedback mechanism .
Only very successful organisms can generate an immune suppressant epidemic .
Small groups or individuals simply get wiped out by things like TB .
Not good . Opportunistic diseases like TB are forced into a more lethal
evolution , while the carriers of HIV live longer . Sooner or later a
super-TB or something like it will evolve and the system will achieve a more
realistic balance .