The even better parasite story is the one that enters into a rat and crosses it's wires so that it is sexually attracted to cat pee. It goes into heat when it sees the cat and naturally gets eaten - the goal was the cat brain the whole time. As heard on NPR http://www.wbur.org/npr/9560048.
It's being run backwards and looks great that way. Imagine playing a couple of pool games and then singling out that one really cool shot you made, and start by saying, "that pool shot is how I play pool."
The parasites did their thing just fine for each individual organism. But that one random one that happened to cross wires in just the right way ended up doing orders of magnitude better. A rare occurrence, but very advantageous once it happens.
Theres actually a relatively simple explanation when you look at the neurobiology. First of all toxo is a parasite that can infect and damage the brain so it makes sense that it could affect behavior. Secondly, the response of fear to cat urine or attraction to female mice is mediated by the same pathways in the brain (the limbic system, which governs emotion, and is closely tied to olfactory input, specially in mice). Making the mice feel attracted to cat urine is actually not that farfetched when you stop to think about it and notice that its just a matter of "flipping a bit" somewhere in the wiring. And since this adaptation leads to a large fitness increase it makes sense that it would be strongly selected for that that the modern version of the parasite would have very specialized mechanisms to take advantage of this quirk in the rat brain.
I get it. The sequence of events seems very analogous to a Rube Goldberg machine, and one would conclude if the sequence of events didn't go as planned the parasite would have been doomed.
My theory is the parasite community for that species can survive other ways just fine, but has stumbled across a scenario that serendipidously gives it an extra boost at the change of reproducing.
I would imagine that most species that benefit from these complex interactions tend to have left-over backups for survival and reproduction. When species over-adapt to specific scenarios, it seems likely they'd become to susceptible extinction or their populations would be periodically thinned out.
