1. Ophiocordyceps One of the better known examples of a living host losing control of its functions via an external parasite, is the ophiocordyceps - a parasitic fungus known for invading and corrupting the brain of individual insects. The fungus really is an incredible, if terrifying, organism. In order to complete its cycle and increase its chances of successful pollination, the ophiocordyceps needs to get to a position high above the detritus covered floor of the African rainforest. It does this in a remarkable way. Spores from the fungus infect insects, for example ants, and essentially re-wire their behavior. The ants no longer follow the instruction of the hive, but instead the spores 'instruct' the ant to climb a stalk and hang from the underside of a leaf about 25 centimeters (10 inches) above the rainforest floor. The spores then kill the ant, using its body to grow a tendril out of its head. After several weeks, the fungus has fully grown and releases its own spores to repeat the cycle. Ophiocordyceps has been known to wipe out entire colonies of ants. Zombifying spores were, of course, the basis of the post-apocalyptic video game The Last of Us, which sees these spores making the jump to humanity. Could it actually happen? Well, I sincerely hope not. The process is truly bizarre and spine-chilling. You can watch it, with added narration from the brilliant David Attenborough, below: 2. Toxoplasma Gondii The toxoplasma gondii is another nasty little parasite which can override the behavior of its host. Toxoplasma gondii can infect essentially any warm-blooded mammal, although it is only known to alter the behavior of rodents. In order to reproduce and complete its life cycle, the parasite must end up within the intestines of a member of the cat family. This is the only place it can sexually reproduce (although it can asexually reproduce within any warm-blooded mammal). Like the ophiocordyceps, it also has a very interesting way of achieving its goal. When it infects a rat, it subtly chances its host behavior to make it less scared of cats, and therefore more likely to be eaten. Tests have shown that infected rats have a decreased aversion to cat urine, while they also have slower reaction times. It is thought these behavioral changes are evolutionary adaptations which allow the parasite a more successful reproductive rate. Could this parasite infect humans? Well, yes. In fact, there's a very good chance you already have it. Estimates suggest one third of the global population is infected with toxoplasma gondii, with the number being generally higher in the Western world. Generally, there appears to be no abnormal effects on infected humans, although some studies have suggested that hosts are more likely to develop neurological disorders such as schizophrenia. 3.Hymenoepimecis argyraphaga Wasps are often scary enough to humans, but if you were an orb weaver spider, you'd be constantly terrified of them - and for good reason. The hymenoepimecis argyraphaga is a Costa Rican pasasitoid wasp which adapts the plesiometa argyra spider's behavior in a fascinating, and disturbing, way. Firstly, an adult female stings the spider to paralyze it, before laying her eggs in the spider's abdomen. The wasp then flies off, leaving its victim to 'recover' and start to move again. However, not long after the eggs hatch, the larva attach to the spider, sucking its blood. After a few weeks, they begin to inject the host with an as-of-yet unknown chemical which alters the behavior of the spider. The larva's chemical actually affects the brain of the spider, making it change the way it makes webs. The spider will be begin to weave a cocoon for the larva, before eventually entombing itself inside. The larva continue to devoir the spider, before developing into adult wasps and heading out to find new victims of their own. 4. Horsehair Worm The horsehair worm, or nematomorpha, is parasitoid animal which also creates 'zombie' insect hosts. Adult horsehair worms are free living, and are known to hang around damp areas such as watering troughs, swimming pools, streams, puddles, and cisterns. Their larvae, however, are parasitic and need to reach water in order to reach maturation. In order to do this, the horsehair worm invades the body of a host insect - often a grasshopper - and essentially makes it suicidal. It compels the host to find water and drown itself in it, returning the larvae to its needed environment. If the host is eaten by a fish upon entering the water, horsehair worms have also been known to survive this process and escape the fish out of any available orifice. Horsehair worms have been also known to infect humans, but only in very rare cases. Luckily, there is no documented case of these infected humans trying to eat brains and/or drown themselves. 5. Creutzfeldt-Jakob Disease Now, we've all heard of Mad Cow Disease (especially if you grew up in Britain in the 1990s), but did you know the virus can also infect a human? If this occurs, it is known as Creutzfeldt-Jakob disease, a debilitating condition which alters the physical and mental behavior of the victim. The disease is caused by prions - misfolded proteins which replicate themselves by converting normal proteins into the same misfolded structure they possess. This leads to a rapid neurodegeneration, with the brain tissue developing holes and a spongy consistency. Although CJD does not lead to particularly violent behavior, its symptoms are bizarrely similar to those of zombies. Firstly, those infected have rapidly progressive dementia, memory loss, hallucinations and personality change - including depression, paranoia and psychosis. This is accompanied by physical problems such as slurred speech, jerky movements, balance problems, and changes in gait and posture. Involuntary movements are also common. CJD is incurable and invariably leads to death within around 6 months - although longer cases have been documented. In a slightly related case, a type of variant CJD known as Kuru disease has been blamed on almost wiping out a cannibalistic tribe in Papua New Guinea. It's important to note that the eating of brains wasn't caused by the disease, instead this was how it was rapidly spread. One way of spreading CJD and its associated transmissible spongiform encephalopathies is through the consumption of infected meat. For example, it is believed Mad Cow Disease (or Bovine Spongiform Encephalopathy) developed because cattle were being fed the ground up remains of other cattle.
Aside from what you've seen in the movies or played on the console, the threat of the living dead walking among us isn't that far-fetched when you consider the natural world. As listed by the website Movie Pilot here are five real life zombie scenarios that could happen: