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Parasite, zombie, leech—these words are often used to describe people in unkind ways. Many of us recoil when ticks, tapeworms, fleas, head lice or bed bugs are even mentioned. Coming across such unwelcome guests—in our hair, on our skin or in our beds—can be a real nightmare.

Some parasites cause horrific , maiming and killing millions of people and wildlife. Others may help boost immunity or .

Parasites are often demonized and misunderstood. But the more we study these oddities and , the more we appreciate their vital roles in ecosystems and . They're essential to life on Earth.

As an ecologist with a focus on wildlife and conservation, I wrote this article to share some of my fascination for parasites and the importance of their extraordinary lives.

What is a parasite?

rely on living organisms for food, to grow and to reproduce.

They can either live on the outside (ectoparasites) or inside (endoparasites) of their hosts. Far from being invited dinner guests, parasites typically turn up of their own accord and feed at the host's expense, consuming part or all of them.

Parasites can live within their host (or hosts) for short or extended periods—in some cases many years—going largely unnoticed. For instance, one man lived with a tapeworm in his brain for more than four years to bear. In other cases, parasites can kill their host.

Perhaps the most gruesome type of parasite, , kill their hosts in order to reproduce. The disturbing is a truly visceral sci-fi example of a parasitoid.

In real life, examples include that first immobilize their spider prey, lay an egg on them, and bury them. Then when the egg hatches, the wasp larvae devour the incapacitated spider. That is, of course, if another animal such as a "" or doesn't intervene.

Parasites are typically much smaller than their hosts. Many are furnished with equipment for latching on and remaining attached, including , and "."

Endoparasites such as tapeworms are often flat, allowing them to live within the tight spaces inside other organisms. The flatworm that lives in gills of some fish must conjoin with another to reach adulthood and reproduce. Once fused, they form a permanent, lifelong bond and mate with each other over many years.

As much as , and this mode of life might have . But parasitism is not solely confined to animals. Many plants, fungi, protists, bacteria and viruses are parasites too.

Parasite powers

The leech scene in the iconic 1986 movie comes back to me every time I walk through a damp forest. The idea of providing a blood meal for another species sparks fear in many people. But leeches may also come to our aid, either by helping to . Their , which are advantageous for medical procedures.

As the blood of leeches contains DNA from their past meals, conservation scientists can use them to .

One of the world's most widespread parasites is Toxoplasma gondii. Some estimates suggest as many as . This parasite's main host is cats, and . House cats are frequently infected, spreading this parasite through their feces.

While many infected people appear to have no symptoms, serious effects can include organ damage, complications with pregnancy or abortion, erratic , , and than unaffected people.

There are potential "benefits" too. Research suggests Toxoplasma infection, which can increase confidence and risk-taking, may even be . Indeed, this same study found that nations with higher rates of toxoplasmosis had a lower proportion of individuals concerned about failure related to new business ventures.

Toxoplasma gondii manipulates its host to increase transmission and continue its life cycle. Infected rodents may become unwitting participants in a game in which they lose their fear of cats and .

Rather than manipulating host behavior, as in the case of fungi that , some parasites cause body malformations. This makes hosts more likely to become prey for subsequent hosts and hence to continue the parasite's life cycle. One of the most striking examples is a (flatworms often known as flukes) that and other amphibians. Extra legs, in some cases several, serve no function and simply impede movement, making it harder to escape predators.

Parasites are fundamental to ecosystems and require conservation

Parasites are a big part of life on Earth. A found the sheer mass of parasites exceeded that of top predators. In particular, the biomass of trematodes was greater than that of birds, fish, burrowing shrimps and polychaetes (marine worms).

Evidence suggests ecosystems than those with fewer parasites. But there is for the amid a growing extinction crisis. So a was proposed in 2020, with priorities including increased data collection and genetic analysis, making conservation assessments, and raising public awareness.

Sadly, parasites can inflict , , suffering, and a heavy death toll. Malaria, (sometimes referred to as snail fever, bilharzia, and Katayama fever), and are just a .

But they also , have , and paradoxically, may in some cases help keep us healthier. Though it may be confronting to admit, we need parasites as much as they need us.

This article is republished from under a Creative Commons license. Read the .