MOST ABUNDANT

Toxoplasma gondii

CDC Public Health Image Library

Some picky parasites live only in certain species, but this protozoan will infect just about any warm-blooded animal. People who have spent time with cats often have it in their bodies. Toxoplasmosis can cause flu-like symptoms but poses little threat to most people. However, it can cause serious problems in people with weak immune systems and can cause birth defects if a pregnant woman acquires it.

DEADLIEST

Plasmodium falciparum

CDC Public Health Image Library

This mosquito-borne parasite causes the most deadly type of malaria, which killed 438,000 people in 2015, according to the WHO’s estimate. This creature is the reason travelers to some regions need to take anti-malarial drugs. It multiplies rapidly in the blood and can cause high fever, anemia and clogged blood vessels.

BEST DISGUISE

Porocephalus

Alejandro Oceguera Figueroa (UNAM)

Pentastomids are called tongue worms because they look like worms (and tongues), but they are actually crustaceans, almost like mini crabs that have adapted to the parasitic lifestyle. (The one in the image, a Porocephalus species, came from the lungs of a green rat snake.) Phillips said she loves the surprise in finding an animal that appears to be one thing but is actually something completely different.

SMALLEST

Early stage of Leishmania

National Institute of Health

Even if it were on display, you couldn’t see the museum’s tiniest specimen without a microscope. It is a very early life stage of the Leishmania species, just 2.5 to 5 microns in length. (There are 25,400 microns in an inch.) Humans get this parasite from sand fly bites, and it creates nasty skin lesions. One type of Leishmania causes lesions on internal organs and can be deadly.

MOST LIKELY TO STAR IN A ZOMBIE FLICK

Nematomorpha

Anna J. Phillips (NMNH, Smithsonian)

Juvenile horsehair worms (or hairworms) infect arthropods, such as crickets and roaches. Once a worm matures inside, it temporarily takes over the host’s brain — or at least its behavior — and compels the insect to fling itself into fresh water. There, the worm emerges from the insect’s cloaca (rear end), and both worm and host go their separate ways.

LONGEST

Diphyllobothrium latum

If you saw this jar on a deli shelf, you’d probably assume it was full of sauerkraut, or maybe cooked egg noodles. You would be wrong. It contains just five tapeworms, suspended in alcohol, each 26 to almost 33 feet long. Phillips pulled these broad fish tapeworms out of the intestines of a stranded bottlenose dolphin in 2014. Even more unnerving, Diphylobothrium latum is one of the three species of tapeworms that humans can get. We acquire it by eating undercooked fish.

BEST PRESENTATION

Nematobibothrioides histoidii