Answer:
Parasites as well as any other organism, need to fullfill their needs of survivability.
Step-by-step explanation:
These needs include having enough energy to survive and pass its genes to the next generation. For that to happen, its life cycle needs to be complete, and for that, the organism the parasite is in cannot die, and that can be a very hard task, once the parasite does not have the knowledge about that, and in lots of times, it kills the organism he infected, dying with it right after because its needs are not going to be fulfilled anymore.
There are several different kinds of parasites, each of them having different life cycles, those can be very hard or easy to accomplish. Another challenge can be the body defenses, that means, chemical reactions could be created by the body to protect itself, killing the parasite.