11.   Memes attach themselves to human hosts and spread, sometimes mutating, sometimes causing mutations.

12.   Memes, Dawkins argued, could even infect their human hosts, like parasites.

13.   Nobody believed that V. cholerae could persist in the environment without a human host, and no cholera outbreaks were occurring anywhere near the Chesapeake.

14.   Previously, scientists thought that a human host was necessary.

15.   Sci-fi thriller starring Donald Sutherland about alien parasites who invade Earth and turn their human hosts into smiling, blank-brained slaves.

16.   She warns that it may be possible for a primate lentivirus to recombine with HIV in the blood of a human host and create an entirely new super-infection.

17.   Since the disease must pass through a human host, it will cease to exist forever when no human being has it, public health officials say.

18.   So biologists are looking for new treatments that will cripple the parasite without harming its human host.

19.   The organism relies on its human host for many essential nutrients and biologists have been unable to ascertain the complete list.

20.   The worm then grows inside its human host for up to a year, sometimes becoming two feet long.