Many of these mutations and changes are actually harmful to the organism and they die or fail to
compete. On rare occasions, however, conditions may prevail that allow a particular variant to have an
advantage over the non mutant strains and therefore they can thrive, and actually predominate. This
phenomenon is called selection pressure. lt can be caused by the actual physical environment the organism
is in, eg heat or cold, but more often it is due to chemicals in the environment, either anti-virals or antibiotics,
or anti-parasitic agents. Obviously, in the presence of these agents, only bacteria that can mutate and overcome
them actually grow and thrive. One can see that by using antibiotics in less than optimal doses for example, one
can easily encourage bacteria to mutate to overcome the antibiotic and gradually replace all the infecting
organisms with ones that are now resistant to that particular antibiotic.
Under other circumstances, chance mutations in bacteria viruses can allow them to adopt to new hosts
in which they previously would not have survived. This is probably likely to be what happened with the
HIV virus. It is now established that there is a Simian (monkey) form of HIV virus, which can actually cause
Simian AIDS. Transmission of this virus to humans would readily have occurred from monkey bites, etc.