Why are parasitic animals dangerous?

Parasitic animals pose varying degrees of danger to their host. Those parasites that do not permanently settle in the host’s body, and partially use its body for food, can carry serious infectious diseases. For example, many mosquitoes, flies, leeches and ticks, which are blood-sucking parasites, are dangerous precisely as spreading infections. And such parasites as tapeworms and flatworms, living in the internal organs of other animals, lead to disruption of their functions and can cause the death of the host.