On the surface, parasites may appear to be simple creatures -- so simple, in fact, that they are not equipped survive without a host. However, many parasites have developed intricate life cycles that may include temporary measures to survive without a host, multiple hosts, and perhaps the freakiest feature of all: the ability to control their host's behavior for the parasite's own benefit.

Ophiocordyceps unilateralis, colloquially known as the "zombie ant fungus," compels infected ants to leave their colonies and lock their jaws around leaves high enough to provide optimal sunlight for the fungus to grow. This has the secondary effect of enabling the fungus to spread its spores onto the nearby ground, usually right onto the colony that the infected abandoned, so that the spores can find new hosts.

Ants seem to be a particular favorite of mind-hijacking parasites. They can also become infected with Dicrocoelium dendriticum, or lancet liver flukes. Lancet liver flukes hatch in the bodies of river snails and are then expelled in snail slime, which an ant might eat. Once these organisms enter the ant's body, one will cling to its brain while the others wait in its abdomen. One thing that differentiates the lancet liver flukes from zombie ant fungus is that ants are only the flukes' intermediate hosts, meaning they can't reproduce inside of ants. Instead, they convince ants to climb to the top of grass blades at night, where they arel likely to be eaten in the early morning by the flukes' definitive hosts: cattle and similar herbivores. Eggs laid in the definitive host will be deposited in excrement to be eaten by snails. This makes the lancet liver fluke complex in two ways: first, it inhabits three hosts over the course of its life cycle, and second, it compels one of these hosts to engage in a fairly unlikely behavior. Not only must the ant climb on top of a blade of grass and stay there outside of the safety of its colony, but it must also know to do this at the correct time of day.

Toxoplasma gondii deploys a similar strategy in rats and mice, decreasing their anxiety and defensive instincts so that they can be eaten by T. gondii's definitive host, cats. The parasite accomplishes this by creating cysts in the rodents' brains that manipulate their immune response. Some scientific findings have linked T. gondii in humans to an increased risk of certain psychiatric disorders, although evidence for this hypothesis has weakened over time.

Although T. gondii may not substantially impact the human mind, the effects of this parasite and others on the minds of host animals raises questions about how the brain and nervous system works. What exploits exist to control other creatures' behaviors, and how can they be hacked into? How complex could the behaviors driven by parasites become in aliens with exploitable nervous systems? What other forms of manipulation might be effective survival strategies for a species, and why might the need for these strategies arise?