Parasitic immatures can complete development in one host. Because they are extremely specialized, they often only attack one or a few closely related species of insect. They DO NOT harm humans or their pets.
Some parasites have extremely complex and wondrous life cycles.
For example, a eucharitid wasp that is a parasite of ants lays her eggs on the leaves of trees. The eggs hatch into a mobile immature that is able to crawl about on the leaf surface. In the spring, worker ants climb into the trees in search of aphids and other insects for food. The parasite larva attaches itself to any worker ant that comes close and, when the worker ant returns to its nest, it carries along the parasite. Once in the nest, the parasite drops off and attaches itself to a larval ant.