Parasites are common. It is estimated that at least half of all living organisms are parasites. Although the term “parasite” was traditionally used only to refer to parasitic organisms such as worms (including hook- and tape-worms) and protozoans (including malaria parasites and Toxoplasma), many researchers now use the term for any organism that lives inside or on a host, and is detrimental to that host. Thus, bacteria (such as the bacterium causing the black plague) and viruses (such as the virus causing yellow fever or smallpox) are also categorized as parasites. Parasites cause threats to humans, agriculture and wild species alike, and a major effort of modern medicine and public health is to reduce parasite infection. Interestingly, removal of some parasites may have negative effects as well, because parasites play important roles in ecosystem functioning and may also prevent allergies in humans. Sound interesting? Check out this BBC article: A World Without Parasites and find out more.

More and more research is showing that parasites manipulate the behavior of their hosts. Why might this be beneficial to the parasite? This behavior increases the parasite’s own survival and transmission to new hosts. One example of a manipulative parasite is shown in this BBC video: Ophiocordyceps. Seems like science fiction doesn’t it? Entomologist David Hughes discusses this further in this radio interview: Zombie Ants Controlled by Fungus. Finding this fascinating? Watch as biologist Kevin Lafferty describes how he and his team discovered how parasites make killifish easy prey for birds in the video: Parasite Manipulators. Finally, watch Carl Zimmer’s TE- Ed lesson: Parasite tales: The jewel wasp’s zombie slave. And you thought zombies were only found in movies and TV shows. Such research often consists of painstaking fieldwork and experiments to determine whether the altered behavior of infected hosts is truly caused by parasites and beneficial to the parasites. Stringent tests are necessary because altered behavior may be a by-product of infection instead.

Do parasites always win? Although parasites often have the upper hand in the struggle with their hosts, they do not always win. Hosts have evolved a wide arsenal of defenses against their parasites, including sophisticated immune systems. In addition, many hosts have evolved behaviors to avoid parasite infection or to treat infections with naturally occurring medicines. Self-medication was originally discovered in large mammals, such as chimpanzees and elephants, but recent studies show the behavior is much more widespread. Learn how butterflies self medicate by watching biologist Jaap de Roode’s TEDYouth talk. How might studying this effect in butterflies help humans in the future? Any ideas?

Want to learn more? Watch Ed Yong’s TED Talk: Zombie roaches and other parasite tales.

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