Vagabond said:
The tapeworm doesn't bother me much, but there are plenty of parasites like the aids virus that end up killing their host. This would seem to me to be contrary to the survival of the fittest rule.
There is no "survival of the fittest rule". This is a creationist straw man. The phrase you want to look up is "natural selection". It's not about "survival" --- it's about differential reproductive success. Ask any salmon.
The other mistake you've made is the fallacy of "group selection". This is quite interesting, so listen carefully.
HIV, like other viruses, works by getting into a cell and hijacking the cell machinery so as to make lots of copies of itself, which then burst the cell and go looking for more cells to infect. HIV is adapted to attack the cells of the immune system, and obviously this is going to have a very bad effect on the patient, as it leaves him prey to opportunistic infections which otherwise he could fight off.
So the virus would do well, you might say (I think this is what you're getting at) if it kept the patient alive as long as possible (to hold on to the territory it already has) and superficially healthy (to increase the chances that he'll spread the virus to other human hosts.)
The fallacy comes in talking about "the" virus, and "it". The question is ---
which virus? In order for them to keep the patient heathy for longer, they'd have to kill fewer cells of his immune system, which means they'd have to reproduce less. But how is this change to take place?
A mutation must occur in one specific virus. Suppose we have one mutant virus, with a mutation which makes it reproduce less abundantly or less quickly then all the other viruses. But this virus and its decendants (which carry the mutation) are competing with other viruses for resources (in particular, cells to infect) and this means that its gene line will soon be history, for such a mutation is by definition bad for the reproductive prospects of the virus carrying it.
If the viruses could all get together and agree all to mutate to be less fecund for the long-term good of viruskind and the little viral civilization they've set up in the patient... then HIV wouldn't kill. But this is the
exact opposite of what the theory of evolution says. It's not about survival, it's about reproduction. It's not about long-term success, it's about short-term success. It's not about the success of groups, it's about the success of genes. It's not directed, it's random. So HIV kills people.
I think you should try to learn more about the theory of evolution before you try to find counterexamples. At the moment, you're criticising what you
think is the theory of evolution and its implications. But you're wrong.