Zombie ant fungus (Ophiocordyceps unilateralis)
Ants are great navigators, following highly efficient paths as they forage for food. But in the rainforests of Thailand, Africa and Brazil, Camponotus leonardi ants get pulled off course by Ophiocordyceps unilateralis, a parasitic fungus (pictured above).
A spore first infects an ant foraging on the rainforest floor, then spends 3-9 days developing inside its body. When the fungus is ready to complete its life cycle, it manipulates the worker to plod blindly away from safety, like a zombie.
A study in 2009 found that the ants always went to similar locations: around 25 cm up a tree, in a spot with just the right amount of humidity for the fungus to grow. The ant then clamps down on a leaf with its mandibles, and dies.
Within 24 hours, fungal threads emerge from the corpse. Finally, a stalk pushes out of the ant and begins raining spores onto the rainforest floor, where they can infect more ants. It's a bit like the chest-bursting scene from Alien, except that the ant is mercifully dead when the fungus explodes out of its head.
Kamikaze horsehair worm (Paragordius tricuspidatus)
One of these worms can grow up to a foot long, and look like a cooked piece of spaghetti. But to get to that point, it needs a house cricket or grasshopper to do its bidding.
The worm coerces the cricket into jumping slap bang into the nearest body of water
First, a tiny horsehair worm larva is eaten by the larva of another insect, such as a mosquito or mayfly. Once this emerges from the water, a cricket or grasshopper will snatch it up. Then the horsehair worm begins to develop inside the cricket in earnest.
But the worm's final stage of development takes place in water. The cricket wouldn't normally swim, or even hang out near water, so the worm must get it there.
By altering the functions of the cricket's central nervous system, the worm coerces it into jumping slap bang into the nearest body of water. The hapless cricket then drowns itself, allowing the horsehair worm to emerge and reproduce.
From the outside, you wouldn't be able to tell if a cricket had been infected, but neurologically, the worm is in control. Ben Hanelt of the University of New Mexico in Albuquerque, who studies the worms, says he has seen a whopping 32 worms pushing themselves out of one luckless host.
Castrator barnacles (Sacculina sp.)
If you think making a cricket believe it can swim is impressive, this parasitic barnacle takes body-snatching to another level.
A Sacculina barnacle enters a host crab by finding a chink in its claw joints. The barnacle sheds its hard shell and squeezes itself in. At this point it looks more like a slug than a typical barnacle stuck to the underside of a boat.
It then sets up home, leeching off the crab's nutrients and turning it into the vehicle that will allow it to reproduce. Once fully-grown, the barnacle looks more like a soft, pulsating egg yolk.
If the crab is female, Sacculina forces it to care for the millions of barnacle larvae as if they were her own. But if the crab is male, it will be feminised in order to do the same thing. Not only is it rendered infertile, it grows a larger abdomen to carry the barnacle's young, its gonads shrink, and it stops developing its fighting claws.
Green-banded broodsac (Leucochloridium paradoxum)
If you see a snail with two beautiful eye stalks, pulsating with emerald- and olive-green stripes and dappled with charcoal grey flecks capped off with a maroon dab, be impressed. You're not just looking at a pimped-out snail, you're looking at a snail infected with a parasitic flatworm.
The green-banded broodsac first squirms its way into the stalks of the snail, so that they look like juicy, pulsing, brightly-coloured caterpillars. This is just the kind of snack nearby birds are in the mood for.
Then the worm manipulates the snail's behaviour. In 2013, Wanda Wesolowska and Tomasz Weslowski of Wroclaw University in Poland found that the infected snails behaved differently from their apparently non-infected counterparts. They positioned themselves in more exposed and better-lit places, situated higher in the vegetation. This probably makes the snails more conspicuous for foraging birds.
Once eaten by the bird, the worm can reproduce, and the cycle continues.
Ladybird parasite (Dinocampus coccinellae)
This wasp needs a host that will protect its eggs from potential predators. So what better bodyguard than an insect with markings that suggest danger?
Ladybirds may seem like the stuff of cartoons and cute lunchboxes, but they can take care of themselves. When disturbed they emit a disgusting poison, and their hard shell with its bright red and black spots warns off predators. But they don't stand a chance against the parasitic wasp, which leaves behind a single egg with one sting.
After the wasp egg hatches, the larva chews through the ladybird's internal tissues before bursting through the abdomen to spin a cocoon between its legs. The ladybird is now a "bodyguard", standing guard over the cocoon. Still alive despite everything, it will thrash and twitch its limbs if a predator approaches. It's not clear why it behaves like this, but it may be triggered by venom left by the larva.
Rather unexpectedly, a 2011 study found that a quarter of the zombie ladybirds survive the assault.
Emerald cockroach wasp (Ampulex compressa)
The emerald cockroach wasp has a metallic body that glows emerald with bright crimson markings on two of its legs. Found in the tropical regions of Asia, Africa and the Pacific islands, it is a beautiful insect, but pity the cockroach that crosses its path.
It is one-sixth the size of a roach, but that doesn't stop it. First it delivers a simple paralysing sting. Then it hijacks the roach's mind, injecting an elixir of neurotransmitters into its brain. This turns the roach into a helpless zombie.
After a quick suck of recharging roach blood, the wasp chews off the roach's antennae and leads it to its nest like a dog on a lead. There it lays its eggs on the roach's abdomen, and barricades it in with pebbles. But the hapless roach doesn't even try to escape, even though it physically could. It just sits there submissively, as the wasp larva eats it alive. Finally the adult wasp bursts out of the cockroach's remains.
Toxoplasmosis (Toxoplasma gondii)
This single-celled creature is perhaps the most famous of all host-manipulating parasites, perhaps because it operates close to home. It mainly infects rats and mice, in order to be eaten by a cat so that it can reproduce.
Between 30 and 60% of people are infected by T. gondii
Infected rats and mice lose their fear of the smell of cats, according to a 2007 study. Instead, they become attracted to a pheromone in the cats' urine. The animal becomes less likely to hide under the floorboards and more likely to sniff around its feline predators, putting the parasite on course for its ultimate destination: the cat's stomach.
Between 30 and 60% of people are infected by T. gondii. But it's less clear that the parasite affects human behaviour. In 2006 Kevin Lafferty of the US Geological Survey in Santa Barbara, California found some evidence of personality changes in people infected by the parasite. So far this is only a correlation, which is far from conclusive. Nevertheless, Lafferty says: "my money is on cause and effect".
Toxoplasmosis is also unusually common in people with schizophrenia, but again it's not clear what that means or how significant it is. "Schizophrenia is a complicated syndrome, perhaps with multiple causes," says Lafferty. He adds that there are plenty of infected people that don't have schizophrenia, and plenty of people with schizophrenia who aren't infected. "Still, I am comfortable in saying that Toxoplasma is a correlated risk factor for schizophrenia."
You might not think of a virus like rabies as a parasite, but to a biologist that's exactly what they are. "I would call rabies and flu parasites, because they generally reduce the fitness of their host, or benefit at the expense of their host," says Levi Morran of Emory University in Atlanta, Georgia.
Rabies is one of the most frightening parasites, because it seems to blur the line between humans and animals. The virus is spread through saliva, usually from a scratch or bite. It makes animals – usually dogs and bats, and occasionally humans – more aggressive, compelling them to spread the virus through biting and scratching.
"Rabies manifests itself with a wide range of neurological signs, including changes in behaviour but also loss of motor control," says parasite expert Andres Gomez of ICF International in Washington, DC. "The latter sometimes include difficulties with swallowing that eventually lead to hunger, hypoglycaemia, and dehydration."
Supposedly, it creates a fear of water, but this is a myth. "Patients have involuntary spasms when trying to drink and later when presented with water," says Gomez. "But it's not fear."
Yes, that's the flu. In 2010 Chris Reiber of Binghamton University in New York and her colleagues found evidence that the influenza virus makes people more sociable.
They found that people given a flu vaccine interacted with significantly more people, and in significantly larger groups, in the 48 hours after being exposed, compared with the 48 hours before. The infected hosts were more likely to head out to bars and parties.
It's only one study, and quite a small one, but it does make a certain sinister sense. It would benefit the virus if its host passed it on to as many people as possible, before the symptoms started and they became bedridden.
Parasites are everywhere. Most species will be living with more than one parasite, and even parasites may have their own parasites. So in some cases, a host may be carrying multiple parasites with different agendas, who must battle for control of the host. This is particularly likely if one parasite is ready to move onto another host, but the other isn't.
It was as if the younger tapeworm wasn't there
To see this happening, Nina Hafer and Manfred Milinski of the Max Planck Institute for Evolutionary Biology in Ploen, Germany infected small crustaceans called copepods (Macrocyclops albidus) with multiple tapeworms (Schistocephalus solidus). These tapeworms ultimately need to move onto a fish host called a stickleback, and to get there they manipulate the copepod's behaviour. It becomes more active, and thus more likely to be spotted and eaten by a stickleback.
If two tapeworms were both ready to move hosts, their effect on the copepod's behaviour was even stronger, suggesting they were working together.
However, if an older tapeworm that wanted to leave was sharing the host with a younger tapeworm that wasn't yet ready, the host copepod still became active. It was as if the younger tapeworm wasn't there.
Hafer and Milinski argue that the older tapeworm was effectively sabotaging its younger competitor.