(ELI5) Why do parasites kill their host in some way if they need a host to live?


Why not benefit the host instead in some way? Killing the host means the parasite also dies.

In: 5

If the parasite has already reproduced or spread, it doesn’t matter if the host dies or not; the parasite has achieved its goal. See also, viruses and bacteria.

>Why not benefit the host instead in some way? Killing the host means the parasite also dies.

Parasites generally do not kill their target hosts. As you noted, that would swiftly remove them from the gene pool.

Parasites will **absolutely** kill other organisms than their specific host if, by some misfortune, they should end up in/on the other organism. It’s not their natural habitat, after all.

E.g. if a parasite that evolved with all the necessary skillsets to feed off of and simultaneously keep alive a massive half-ton meat tank of a mammal somehow ends up in a cat and also somehow survives then that’s bad news for the cat. There will be a misunderstanding between the parasite and the cat to the detriment of both of them.

If a parasitic animal (such as a wasp) is adapted to kill its host, it is called a “parasitoid,” the wasp larvae grows to adulthood and survives the death of its host.

True parasites need to reproduce before the host dies to be successful.

When a parasite benefits its host (mycorrhizal fungi for example) then by definition it is not a parasite, it is a commensal or mutualistic partner.

Analogously, why do humans destroy the environment if they need the environment to live?

Organisms don’t think that far ahead.

There are many ways for a parasite to benefit its host. For example, some parasites provide nutrients to their hosts, while others help the host to break down food more efficiently. Some parasites even provide protection to their hosts against other predators. In return, the host provides a safe environment and food for the parasite.