It's parasites week here at mental_floss, which we kicked off two days ago with the release of our newest video, "Attack of the Parasites." Now, while it's true that there are all kinds of compelling reasons to avoid becoming infested with parasites -- especially by the kinds that cause things like river blindness and elephantiasis of the genitalia -- there are also a few reasons to make nice with our tiny, opportunistic compatriots. Here are four.

1. Pseudacteon, the "decapitating fly"

Nobody likes fire ants. Where I grew up in Southern Florida, they were a major nemesis to unfettered, shoeless outdoor romping; one false step could mean a foot covered in biting, welt-inducing fire ants. In fact, ever since they were accidentally imported from South America via cargo ship in the 1930s, they've spread like an annoying, predator-less plague from coast to coast, infesting at least 18 states. So far, the only measures introduced to deal with them have been stopgap at best -- sprinkling poison on the mounds, for instance. That is, until now.

Meet Pseudacteon, a parasitic fly from South America. Pseudacteon, like most living creatures, loves to reproduce. The important distinction between it and other flies, however, is where Pseudacteon lays its eggs: inside the heads of fire ants. Get ready for an awesomely grody cycle-of-life: the larvae develop by feeding on the muscle and nervous tissue in the head. After about two weeks, they cause the ant's head to fall off by releasing an enzyme that dissolves the membrane attaching the ant's head to its body. The fly pupates in the detached head capsule (pictured above!). Hence: the decapitating fly! Currently, industrious agricultural scholars are importing them from South America in the hopes that they'll do rid us of our fire ants. Godspeed, little decapitators.

2. Worm therapy

A rare bit of good news from the world of gastrointestinal maladies: swallowing the eggs of a certain type of parasite worm may actually help cure Crohn's disease. The basic theory is this: in countries where people's exposure to parasites is high, disorders of the immune system like Crohn's, multiple sclerosis and type 1 diabetes are rare. In ultra-sanitized industrialized nations like the U.S., they're common. According to researchers, we may have hygiened our way into a few diseases that certain types of parasites actually help prevent, and by re-introducing them into the body -- that is, with "worm therapy" -- we may actually be able to combat those conditions.

Specifically, treatment with the eggs of the intestinal parasite helminth Trichuris suis may be a safe and effective treatment for Crohn's disease, according to the results of a small, preliminary trial conducted at the University of Iowa in Iowa City. T. suis is not a natural human parasite, but primarily affects pigs.

3. The blepharipa schineri fly

If swallowing worm eggs isn't your thing, then maybe you can get behind this: the fuzzy, half-inch-long blepharipa schineri does for the gypsy moth what the decapitating fly does for the fire ant. (Well, it doesn't actually decapitate the moth, but the end result is the same: no more moths. It achieves this by laying eggs in pre-moth gypsy caterpillars, where they hatch and prevent them from growing into moths.)

What's so terrible about gypsy moths? Just ask a park ranger: gypsy moths eat the leaves of some of our most popular shrubs and trees, from oak to beech to poplars. In the Eastern U.S., the moths are well-established over at least 60 million acres, defoliating and killing hundreds of varieties of trees. If you don't like the sound of that, you just might like the sound of what the fly does to the moth:

A parasite of the gypsy moth, the B. schineri fly kills unlucky gypsy moth caterpillars that accidentally eat fly eggs while munching on leaves. A gray-black speck about the size of a thumbtack point, the fly egg hatches inside the caterpillar's stomach. The transparent B. schineri maggot that emerges will later poke holes in the caterpillar's gut, then wriggle its way to the nerve cord that runs the length of the body. While the gypsy moth caterpillar transforms into a pupa--a pre-moth that slumbers in a silky, loosely woven cradle--the B. schineri maggot feeds on the host's innards, slowly killing it.

Gross? Sure. Useful? Definitely. Lucky for us, moths are this fly's only breeding ground.

4. It Makes the Ladies Happy

Quite literally: the Toxoplasma parasite, found primarily in cat feces, is known to have a mood-altering effect on humans, which differs depending on the sex of the human. In a study conducted by parasitologist Jaroslav Flegr of Charles University in Prague, some infected women were likely to become more outgoing and warm-hearted, whereas some infected men became more jealous and suspicious. (Not everyone is mood-altered by cat poo; it's a small, though statistically significant, percentage.) When it's not changing our moods, the Toxoplasma is trying to spread itself by making rats who come into contact with it act fearless, which makes it easier for them to be caught by cats (who then become infected, etc).