According to McNeill, the malaria parasites find new hosts through adaptations that allow them to move from mosquito to human and back again. 

When the parasite is inside the mosquito, it behaves in a different way than it does while it is in a human host.  The plasmodium stays in the mosquito’s stomach and does not do anything that harms the mosquito.  Eventually, the parasites reproduce and move back to parts of the mosquito from which they can be reinjected into human beings.  It is important for the parasite to be relatively benign while it is in the mosquito.  It relies on the mosquito to fly around and inject it into new hosts.  If it were to make the mosquito too sick to fly, it would not reach the new hosts.

Once the parasite is in the human being, it behaves differently.  Here, it periodically destroys the red corpuscles it is living in.  When it does this, the person becomes very sick, but the plasmodia are moving freely in the blood stream and can easily be sucked up by a mosquito and transferred to a new host.

Thus, by behaving differently in mosquitos and in humans, plasmodia are able to find new hosts.