Pathogens (viruses, bacteria and parasites) can cause a series of reactions in their hosts, which is another name for the organisms they infect. Many of these reactions are general reactions, and are an attempt by the host's immune system to get rid of the parasite. For example: if a common cold, a rhinovirus, infects your nasal passages, your nose will start to run, which is your body's way of washing out the virus. A chronic runny nose can cause your sinuses to become stuffed up, which can cause a headache or an earache or even an eye ache. As your immune system fights off the virus, you become tired or develop a fever. As your sinuses become irritated, you start to sneeze. As the mucus from your nasal passages runs down your throat, you get a sore throat and sometimes a cough. The sneezing, coughing and excess mucus then spread the cold on to the next person. All this because your body recognized an intruder in your nose and tried to get rid of it. The common cold is very simple in its effect on your body: the effects are all linked back to the site of infection. Staph, a bacterium that causes skin infections, is similar, as is Guardia, a parasite that infects your small intestine. Some pathogens can cause much more complicated reactions in the host. Some viruses can cause cancer, for example. And some parasites can even change the behavior of their hosts.

It sounds like the parasite you heard about one of these types of pathogens. I did a little reading and it looks like the Gordian worm parasite is like many parasites: the adults swim around in fresh water (ponds/lakes/streams), doing no harm. In fact, the adults don't even eat, but live off stored resources they built up when they were young. Their one activity as an adult is to mate and lay eggs. When the right insect eats their eggs, the eggs hatch inside the insect's gut. When the worm is young it is parasitic, living off the food inside the insect's gut and eventually the entire gut. Also like many parasites, once inside their hosts, the young worms go through a complicated set of stages before they become free-swimming, harmless adults again.

Most parasites life styles are complicated, and involve multiple hosts. Malaria is caused by a parasite which has two hosts: mosquitoes and humans. Gordian worms also have two hosts. Both are insects. When a second host feeds on the first host, which has the younger parasites in its gut, the second host gets infected with the parasites as well, and the parasites continue to mature inside the gut of the second host, getting closer and closer to an adult. Unlike many parasites, the Gordian worms remain inside the gut of their insect hosts, both the first and second hosts, eventually completely consuming the entire gut, which kills the insect. Before the second insect host dies, the young worms need to get deposited in fresh water so they can continue to live as adults and start the whole mating process again. Some insects fly, which makes it uncertain that the host will be near fresh water when the worms are ready to bust out of the gut and turn into adults. To make sure the insect is near water, the worms change the behavior of the second host, thus ensuring their survival.

Sure, it sounds easy in theory, but how do the parasites do this in practice? This is actually a well-studied branch of a science called parasitology, which is the study of parasite biology. It turns out MANY parasites change the behavior of their hosts, in many different ways. By changing the behavior of the host, it makes it more likely the parasite will survive. The change in behavior may ensure the second host of the Gordian worm, the insect with the mature parasites in its gut, moves toward water, or the change in behavior may ensure the first host of the Gordian worm, the insect with the immature parasites, acts in such a way that it is sure to get eaten, so that the parasite is transferred between hosts. How parasites do this exactly is still being studied, and probably varies quite a bit among parasites. Some parasites change the sexual or social behavior of their hosts. Others make them more or less active. Still other parasites make their hosts seek light, or climb higher on stalks, or actually change color or size. These probably involve a combination of effects on metabolism, the production of chemical messengers (e.g. hormones, neurotransmitters) or even brain damage to specific parts of the brain. Rabies destroys the part of the brain that regulates fear and aggression, so infected animals are virtually fearless, and are much more likely to attack other organisms, even ones that are bigger than they are. For a list of known mechanisms for how parasites affect the behavior of their hosts, check out:

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There are several ways, but generally speaking this means fussing with the instincts of the creature in question. This can be done by altering its hormonal or other chemical balance, or by wiring into its nervous system. I don't know how this particular parasite does it, but it obviously disorients the cricket somehow such that it jumps into the water - which drowns it, of course, and allows the parasite to complete its life cycle.

Parasites generally induce behavior that furthers their propagation. This is why colds make you sneeze.