A parasite, as per the Oxford Dictionary, is - 'an organism which lives in or on another organism (its host) and benefits by deriving nutrients at the other's expense'. Tapeworms, leeches, lice, and bedbugs are common examples of these nasty creatures afflicting humans. They can reduce nutrient absorption, cause pain and discomfort, or act as carriers of fatal diseases. Besides that, some parasites are also capable of manipulating their hosts in bizarre ways. They might do this for protection, food for their young ones, or to get transferred to their next host. Whatever the reason, these creatures just might make you feel grateful that you are a higher-order animal, immune to them. Or maybe not... Taxoplasma gondii, one of the most common parasites in the world, can infect almost all warm-blooded species, including humans. However, it can complete its life cycle and reproduce sexually only in felines, and hence alters the behavior of its host to increase interaction with cats. For example, it might make rats and mice 'friendlier' towards cats - their natural predators. What is more horrifying is its prevalence - up to 60 million people in the US alone have been estimated to be infected by this parasite, and a person can get infected by simple, everyday things like eating unwashed fruits or vegetables, undercooked meat, or handling cat litter. In humans, it is linked to subtle changes in personality and increase in neurotic tendencies. Scientist have also found a link between the presence of this parasite and suicidal tendencies, schizophrenia, and traffic accidents. It is speculated that the bacteria releases dopamine and alters the hormones of its host. What could be more horrifying than a brain-controlling parasite? How about a parasite that has the power to annihilate all males of a species and make its host completely dependent on it. Wolbachia is a very prevalent parasite, capable of infecting up to 60% of insect species worldwide. It has the ability to make the males infertile, or transform them into females or pseudo-females, or even kill all males in a brood. It does this as it can only spread through females; although males can and do get infected, they cannot pass the infection to their offspring. Another intriguing aspect of this parasite is that it makes the infected insects better resistant to disease and pesticides, often resulting in complete dependence of the species on Wolbachia for survival and reproduction. The Ribeiroia ondatrae is found in freshwater, where it enters the amphibians in their larval stage - for example, a tadpole. When the tadpole begins its transformation, the parasite affects the limb sites and disrupts the cells, resulting in a 'mutant' frog with missing, deformed, or extra limbs. These frogs are easier to catch and so an easy meal for birds - the host in which Ribeiroia reproduce and rely on to get their spawns spread far and wide. Sacculina carcini are a type of barnacles which parasitize and sterilize crabs. First, the female barnacle will find a crab and enter its body in the form of embryonic cells. Once inside, it spreads root-like tendrils all over the crab, absorbing nutrients from it, and stopping the crab from molting - which means the crab stays small in size. She will also make a 'home' for herself - a sac on the underside of the crab, where the crab's reproductive organs usually are. This also means that the crab - male or female, is rendered infertile. If the crab is a male, the parasite affects his hormones and changes his physiology as well as behavior, feminizing him. Then comes the male barnacle, who fertilizes the female and the resulting eggs are cared for by the crab, all the while being eaten by the barnacle family. Plesiometa argyra is a little spider with a pearlescent back, and yellow, white, and black coloration, found in North America usually constructing the normal, round webs used for trapping its food. But when the Hymenoepimecis argyraphaga wasp lays eggs on it, the pattern of its web completely changes. Once the eggs have hatched, the larvae feast on spider-juices for about two weeks, then when it has almost run dry, the larvae makes the spider build a completely different web - a water-proof, wind-resistant refuge, suspended in mid-air to keep them safe from predators. The Glyptapanteles wasp usually lays its eggs in a young caterpillar. On the outside, the caterpillar behaves normally, but inside, the Glyptapanteles eggs hatch and feed on its body fluids. Once fully grown, the larvae emerge from the half-dead caterpillar and begin their transformation into a pupa. The caterpillar, at this stage, stands guard over the pupae; thrashing wildly at any hint of danger. The caterpillar stays in this manner, without feeding or moving away, and ultimately dies due to starvation, usually just after the wasps emerge from the pupae. The Lancet Liver Fluke is a perplexing organism - needing to go through at least three hosts to successfully complete its life-cycle. The first host is either a cow, sheep, or goat; then a snail, and finally an ant. The fluke's movement in the first two hosts is relatively simple, but it adopts a rather complex process to get transferred from the ant to the grazing animal. The cattle won't normally be able to eat an ant, as it is small, quick and not really what they like to eat. So, the fluke makes the ant clamp on to a blade of grass, waiting for a grazing animal to eat it, every evening. When morning comes, the ant will behave normally, and continue with its usual activities, day after day, till either the ant gets eaten or perishes in some other manner. It manages to manipulate the ant in this manner is by invading its brain - the first cercaria (lancet fluke larva) to reach its brain, gets transformed into a brainworm, and changes its behavior. Evolving since millions of years, these mind-controlling parasites have formed complex, and sometimes symbiotic relationships with their hosts; like the Wolbachia. They might seem right out of someone's worst nightmares with their zombifying and mutational abilities, but these creatures are very common and all around us, and maybe even inside us.