Our general understanding of parasites is that they are generally small organisms that infect specific animals or plants. They differ from predators in that they do not kill or permanently immobilize their victims immediately. Having infected their usual host, the parasite’s growth and reproduction, on or within their host, may or may not cause disease. But one way or another they mature and shed their numerous eggs or offspring through lungs, feces, or skin into the surrounding air, water or soil with the expectation that at least a few of their eggs or offspring may, by chance, be taken up by another suitable host.
We have learned, however, that there are a number of parasites whose behavior or effect on their host’s behavior is such that there will be a much greater probability of their offspring finding the appropriate hosts. One such parasite is a fungus that infects carpenter ants. As this fungus matures in the ant it causes it to leave its colony, climb a tree, and find the underside of a leaf or an overhanging twig. Thereupon it fixes its sharp mandibles firmly into the twig or leaf and dies. The fungus then completes its life cycle by sending out fruiting bodies from various parts of its victim which, in turn, shed their spores into the air to land upon the very places where more such ants are likely to be foraging. A close analogy of this behavior occurs in humans infected with a cold or flu virus which, by greatly irritating the nasal passages and lungs of the infected individual, causes sneezing and coughing, enabling the newly-shed mature virus to be widely spread to other susceptible humans.
A protozoan (a single-celled animal), Toxoplasmosis gondii, which can cause severe illness in the human fetus or in humans whose immune system is compromised, infects a variety of different mammals but can only reproduce in cats. However, when it infects rats it alters their brain in such a way as to remove their fear of cats. In this way the rats are more easily caught and eaten by the cats, thus returning the parasite to the one host in which is able to reproduce. There is also a flatworm parasite that matures by sequentially infecting fish followed by birds. Its presence in its first host, killifish, causes the unsuspecting fish to flash their bright sides in the sun as they swim thereby making it easier for the herons that feed on this fish to find them. Once in the bird, the flatworm can mature and shed its eggs back into the water. A parasitic wasp exists which preys on cockroaches by injecting them with a toxin that makes them docile and susceptible to being herded into the wasp’s nest to be eaten by the wasp’s young. More recently it has been learned that a virus, carried by cricket-eating lizards, infects crickets and causes them to mate more frequently and energetically in spite of the fact that their reproductive tissues become depleted of nutrients by the replicating virus. These frequently-mating crickets thus spread the virus widely in the cricket population. These infected crickets in turn spread the virus back to the lizards that eat them, thus ensuring the virus of yet more lizards in which to multiply. Finally, as has been learned in other contexts, not all bacteria of the same species may behave identically. An important example of this occurs with the typhoid causing bacteria which, upon infection, responds in one of two ways: some replicate causing typhoid fever while others go into a persistent non-replicating dormant state. These latter bacteria, protected from antibiotics because they are not growing and dividing, can either break out at a later time causing a relapse in the illness or be transferred by contact or through feces to other, more susceptible, humans.