Can you be a host and a parasite at the same time? The answer is a stomach-turning “yes.” Like grotesque Matryoska dolls, naturalists sometimes find parasites within parasites.

1. "ANTI-ZOMBIE FUNGUS" FUNGUS 

Brain-controlling parasites sound like the villains of a cheap sci-fi movie. But make no mistake: They’re all too real.  Just ask an ant.

Members of the Ophiocordyceps fungal genus have evolved to attack a specific type of ant. In order to grow properly, these fungi need a patch of earth that meets several very specific conditions (perfect temperature, humidity, distance from the ground, etc.). But it needs transportation to get there.

When the right Ophiocordyceps encounters the right host, things get weird fast. Fungal spores infect the ant's brain, and literally control its mind. It drives the ant to prime Ophiocordyceps real estate, where the ant bites down onto a leaf and dies. After the ant expires, thread-like filaments spread through its body. These soon breach the exoskeleton and, before long, new Ophiocordyceps spores rupture the ant’s head, ready to repeat this cycle of mayhem.

A third player in this Jacobean drama was revealed in 2012. A team led by Penn State biologist David Hughes discovered a second fungus—one which saves countless ants from mental enslavement. This currently nameless organism turns the tables and makes a host out of Ophiocordyceps itself. “The hyperparasitic fungus effectively castrates the zombie-ant fungus so it cannot spread its spores,” Hughes says.

2. DOUBLE-PORED DOG TAPEWORM (DIPLYDIUM CANIUM)

Man’s best friend is a four-legged flea magnet. Multiple species target canines, with the aptly named dog flea (Ctenocephalides canis) being the most widespread. It also can infect the skin of felines and—occasionally—humans. 

Apart from being annoying, the flea can be a Trojan horse for a far more dangerous parasite. Larval fleas frequently eat the eggs of double-pored dog tapeworms. These hatch into cysticercoids (tapeworm larvae), which bide their time inside the flea’s intestines while the arthropod grows up. Should a dog, cat, or human accidentally swallow an infected flea, the cysticercoid will break out and develop into a mature, full-length tapeworm. Upon becoming sexually active, this segmented creature releases eggs that the host later poops out. Fleas then gobble them up, and the cycle repeats itself.

3. ASAPHES CALIFORNICUS WASP

Any parasitic organism that invades another one is called a “hyperparasite.” And then we have A. californicus, which counts as a “hyper-hyperparasite.” Pea aphids (Acyrthosiphon pisum) are red and green sap-suckers that can be found throughout much of Europe and North America. Commonly regarded as pests, the bugs are vulnerable to Aphidius smithi: a wasp whose larvae live inside them and mooch off the aphids’ food supply.

Life isn’t exactly a cakewalk for these freeloaders, however. Aphidius smithi larvae can themselves become infected by the larvae of another wasp: Alloxysta victrix. Those, in turn, will sometimes harbor A. californicus larvae. So, to recap, a single aphid might well have a parasite that has a parasite that has a parasite. Isn’t nature delightful?

4. DACTYLARIA THAUMASIA FUNGUS

As the old cliché goes, the enemy of your enemy is your friend … sometimes. Farmers the world over dread an outbreak of the cereal cyst nematode (Heterodera avenae). Under the right conditions, it’ll colonize wheat, oats, barley, and other profitable crops. Once inside, the nematode has a nasty habit of knotting up roots, which may ultimately kill the host. Economically, the consequences are often dire: In 1988, this organism single-handedly caused wheat productivity in Pakistan to drop by an estimated 15 to 20 percent.

Yet in our war against the menace, we’ve found an unlikely ally. Dactylaria thaumasia is a parasitic fungi that severely weakens nemotodes, putting a check on their destructive talents. Hence, as a precautionary measure, many crop-growers now plant the fungus directly into their soil.  

5. WOLBACHIA BACTERIA

This is another set of cascading parasitic relationships. From the onset, this bacterium stacks the deck in its favor. Wolbachia is a genus of microscopic marauders that plagues female wasps, which inject their larvae into young botflies (themselves parasitic). The bacteria arrest the wasp’s ability to produce male offspring. By making sure that wasp populations are kept disproportionately female-heavy, Wolbachia is able to maximize the number of larvae it can infect.