It's bothered me for a while now. I understand why a parasitoid would do this, as it only temporarily requires the host, and that not all parasites kill their hosts.
There seems to be no evolutionary advantage in killing a host, because the parasite relies on the host for resources. Yet some organisms, usually microscopic pathogens, seem to fatally damage the host with no immediate benefit to themselves. Why is this? Won't it be better to have a commensalism relationship with the host so both exist healthily?
I know this is asking for a broad soft answer, but I don't want the question to get bogged down by a specific species, although it would be good to have examples.