Why do parasites sometimes kill their hosts?

Question

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.

Answer 1

I can think of several (non-exclusive and probably non-exhaustive) hypotheses:

1. Maladaptive. It is maladaptive. Maybe because it is a virus coming from another species (e.g. SIV and HIV) or because it is not adapted to our modern lifespan.

2. Benefit of the host immune system to the parasite. Some parasites might benefit from host immune defence (e.g. sneezing helps bacteria to spread) and death is a consequence of the selection for increasing disease symptoms to the host.

3. Pathogen's and host's fate. The pathogen's fate is not linked to the host's fate. This is especially true if the pathogens spread well (easily jump from one host to another) I guess or for parasitoïd because the pathogen leaves its parasitic life after the death of its single host.

4. intra- and inter-host selection. Among hosts, there might have a selection for reducing resource consumptions and therefore for decreasing the probability for the host to die. But within a host (among individuals of a parasite population or even among parasite species), it is a prisoner's dilemma (tragedy of the commons). The more you invest in foraging, the more competitive you are. This might not be adaptative at the population level but it is at the individual level. We might think of this prisoner's dilemma happening among individuals of a single parasite species or among several parasite species. You may want to have a look at the work of Martin Nowak on the subject incl. Consider for example his book; Evolutionary Dynamics.

5. Consequence of harming on trade-offs. Harming an individual will cause him not to spend energy into reproduction in order that more energy is available for the pathogen use. Moreover, by causing symptoms, the infected individual will be sexually less attractive and will again have less opportunity to spend energy into reproduction.

6. The pathogen does not mean much harm. When fighting pathogens, many of the symptoms we experience (such as fever typically) are actually caused by the immune system and not the pathogen itself. A maladaptive overreaction of the immune system can cause our death.

The book Why we get sick by Randolph Nesse and George Williams, will probably interest you.

Answer 2

Some parasites aren't natural hosts of humans, instead we are an accidental host in which they cannot replicate. Zoonoses such as this can be quite fatal as the parasite is not adapted to us. Although viruses are often separated, they are parasites and you can see just in influenza that the animal derived flus are more fatal than those that are well adapted to humans.

Otherwise because there is no need for the host they can be fatal. If they have finished their round of replication or require death of the host to be able to spread. Still this is less common than simply a badly adapted parasite.

HPV causes cervical cancers. It is adapted to increase replication of its host cell which is obviously beneficial, but this also kills the host in the long term. Thus some viruses are not yet adapted to the new longer lives we live, but may never do so due to less of a survival pressure hence vaccines.

Please feel free to ask questions.

Answer 3

This is related to Remi.b's answer, but a common reason is that sometimes parasites/pathogens actually need to kill the host to spread. A lot of viruses, in particular phages, are only released from their host by bursting out of the cell. If the parasite can't leave the host, there's no point in any of it, and death may be a good way to achieve that. A number of diarrheal diseases in humans cause death because massive diarrhea (aka water loss) is how they spread.