Now that the world has gotten smaller, airplanes move from one country to another, picking up passengers and exchanging packages. We have set them free. Canals constructed for shipping in the early part of the century allowed lampreys, normally restricted to the ocean, to float into the Great Lakes. By 1960 trout populations had plummeted and once prosperous fishing towns became economically depressed. Planes fall from the sky just as lampreys cup their round incisor mouths around the body of the trout.
Parasites are nothing to fear.
As humans we are all half parasite. No one is purely human. We are filled with the blood and infections of the entire human race. With time we will evolve to accept our hybridity. Birds fall from the sky and land in playgrounds where children, witnessing the descent, will forever after believe that it is natural and right for birds to remain for a moment suspended airborne and then fall down dead.