These are my notes for a TED talk about parasites called "Suicidal wasps, zombie roaches and other parasite tales" by Ed Yong.
Ed Young talks about ways that parasites can alter many attributes of their hosts, including behavior, to serve the needs of the parasite rather than the host. One example is that when a certain kind of tapeworm infects a brine shrimp, it turns the shrimp red and causes it to gather in large groups. This allows the shrimp to be easily seen and caught by flamingos, who are the only hosts in which this tapeworm can reproduce. There are many cases like this in which a parasite makes it's host behave in ways that are dangerous and even fatal to it but serve the needs of the parasite. He gives an example of a kind of worm that infects a cricket, and releases chemicals to cause the cricket to jump into water and drown. Once it's drowned the worm is free to leave and look for a mate.
Since parasites are hidden through much of their lives, it is easy to underestimate their numbers. He mentions research in which it was found that the biomass of a particular kind of worm in a particular estuary was the same as that of all of the fish in the same estuary, and 3 to 9 times that of the birds. Because of the sheer volume of parasites, manipulation is not unusual in the natural world- it is a "critical and common part of the world around us".
Scientists have found hundreds of examples of parasites capable of manipulation, and is now beginning to understand some of the specific means by which they accomplish this. There is a wasp that has a specialized feeler so that it can reach into the head and locate the brain of it's victim (a roach), and it carefully injects a very specific type of neurotoxin into two very specific regions of the roach's brain.
This raises the question of whether there are parasites that are able to manipulate humans in the same way. This question is deeply unsettling since as humans we tend to think of ourselves as in control of our own behavior, pretty much all the time. One candidate is toxoplasma gondii, which is capable of infecting mammals in general but can only reproduce inside of a cat. If a mouse or rat becomes infected with toxo it will not only avoid cats but will seek them out. The cat then kills and eats it, and the parasite is able to reproduce. They are not entirely sure how this works, but they do know that toxo makes an enzyme that makes dopamine; we also know that it targets a part of the rodent brain involved in sexual arousal. Estimates are that 1 in 3 people around the world is infected with toxo. For the most part, toxo is dormant in humans and it is not immediately obvious if someone is infected. However some small signs have been found, such as a slightly increased likelihood of being in car accidents (this is hypothesized to be due to slowed reaction times). There is also some evidence that people with schizophrenia are more likely to be infected. Although we don't know exactly how or how many parasites such as toxo are able to influence us humans, given how widespread this phenomenon is in the world it is extremely unlikely that humans would be the one exception, the one animal not able to be influenced by parasites. He adds that viruses and fungi can also be considered parasites, which raises a lot of questions about human behavior and infections. These questions are difficult and we don't have a lot of answers but they do seem like very important things to consider.
This is another video on the topic that presents some additional information:
