Once upon a time in Western Africa, some hunters killed and ate a chimpanzee.
There was nothing out of the ordinary about that. The flesh of primates has always been a delicacy in the region.
Trouble was, this chimpanzee was different.
It had the Human Immunodeficiency Virus (HIV) lurking in its bloodstream, waiting to pass from primate to human.
Immune, itself, to the effects of the virus, the chimp nonetheless was a carrier, and one or more of the hunters became infected through contact with the blood of the animal.
The rest is history.
Out of such an innocuous event, those hunters in an African forest unwittingly played host to a hitherto unknown submicroscopic particle that would bring about sickness and death across the globe and threaten the very existence of humankind.
Had it not been for the intervention of antiretroviral therapy, the scourge of HIV would have killed many times the number of people it has up until now.
Think about it for a moment. If an affliction like HIV could pop up seemingly out of nowhere, and cause millions of deaths worldwide, what other threats to human existence might be lurking, waiting to pounce?
Let’s take a look at some of the nasty things we know of that nature can, has, and does unleash on humankind and other life forms.
One such is a parasitic fly, aptly dubbed the zombie fly, which goes around infecting bees, turning them into zombies in an insect version of the fictional “zombie apocalypse” syndrome where undead flesh-hungry corpses roam the world infecting whomever they bite, the victims of which themselves turn into zombies a day or so later, exponentially creating more zombies until they outnumber the living.
What the flies do is home in on unsuspecting bees going about their daily forage and inject their larvae into the bees’ abdomen midair. In due course, the larvae hatch into maggots which then feed on the bees from the inside out, causing a slow and excruciatingly painful death.
Before they die, the bees begin to exhibit zombie-like tendencies such as flying at night, which bees almost never do, and behaving erratically until they drop dead. The maggots then turn into flies and the cycle begins all over again.
Some parasites play the game differently, choosing not to kill their hosts, but rather to manipulate their hosts’ behavior in ways beneficial to themselves.
A parasite known as Dicrocoelium dentriticum, for example, begins its cycle inside a snail, which duly excretes the worm to be eaten by an ant. At this point, the worm makes its way to the ant’s brain where it somehow rewires the neurons. This turns the ant into a willing zombie, which, at the worm’s command, climbs to the top of a blade of grass where it waits for a grazing sheep to come along and consume it.
Safely inside the sheep’s stomach the worm lays its eggs, which then leave the sheep via its excrement, a ready-made meal for snails. Finally, to complete the cycle, the eggs exit the snails mixed in their excrement for more unfortunate ants to feed upon.
Then there are those delightful parasitoid wasps which lay their eggs inside spiders. When the eggs hatch, the offspring slowly devour their way out. But not before they modify their hosts’ brain so they spin webs designed to support the cocoons when they pupate.
Apart from HIV, which isn’t actually a parasite, the above nasties target only nonhuman life forms. But there are parasites that will go for anything that’s on offer, humans included.
Toxoplasma gondii (Toxo for short) is such a parasite. And this little moocher loves cats. In fact the survival of its species depends upon cats. That’s because it can only reproduce in a cat’s gut. So Toxo likes to take good care of its feline host by providing it with an easily accessible food supply.
The way Toxo does this is really ingenious.
What the parasite does is take a sojourn into the outside world when the cat defecates. It then gets picked up from the soil by scavenging animals, including its intended target: rats.
Once inside the gut of a rat, Toxo makes its way to the animals brain. The parasite then gets to work modifying the rat’s behavior to make it attracted to the odor of cat’s urine, and also to make it less afraid of cats.
In this way Toxo provides cats with the means to easily catch rats to feed upon, while at the same time providing itself with the means to get back into a cat’s gut.
Trouble is, humans are also exposed to infection by the Toxo parasite, particularly from contact with cat litter boxes; and from the food they eat, such as fresh raw salads, under-cooked meat, and even from drinking water contaminated with cats’ faces.
As in the case of rats, when Toxo parasites gets into a human host they lodge in the brain (and other tissues) and get straight to work making behavioral modifications which can cause schizophrenia and suicidal tendencies.
Parasites are cunning and ruthless and care nothing for their hosts. In those cases where they appear to be serving their hosts’ needs, it’s only because they are actually serving themselves in the long run.
And it’s anyone’s guess what especially nasty parasites might be lurking out there, just waiting for the opportunity to strike.