Parasites don't kill their hosts because they often require a living host to feed, thrive and reproduce, according to Medical News Today. Since parasites have to find a new host when the current one dies, it's in the parasite's best interest to keep the host alive in order to absorb as many nutrients as possible from its system.
While most parasites do not directly threaten the life of their hosts, the energy and nutrients they zap can indirectly cause the death of the host human or animal. A parasite infestation can leave the host open to opportunistic illnesses or infections, and if too many parasites attempt to invade a single target, the organisms can make it impossible for the host to support them all.
Other types of parasites may give off toxins that can negatively affect the host's health. A few parasites have been known to influence their hosts' behaviors in order to further their own life cycles, according to LiveScience. For example, toxoplasma gondii, a common parasite found in rats, can alter the mind of infected rats by driving them to areas frequented by cats in order to move up the food chain, and hairworms can induce grasshoppers to leap into water so that the parasite's young can swim away and continue their growth cycle.