ProtozoaFoodPets & animalsSoil
Toxoplasma
Toxoplasma gondii
Lives in brain and muscle for life. The parasite behind the fearless-rat story.
Full file
- How common
- More than 40 million people in the U.S. may carry it (CDC). Worldwide, roughly 1 in 3 people.
- What it does
- Most people never notice. It forms dormant cysts in the brain, eyes and muscles that stay for life. It can be serious for unborn babies and anyone with a weak immune system.
- Nerves & brain
- Cysts sit inside brain cells. Studies link infection to slower reaction times and higher rates of some mental illnesses, though cause and effect isn't proven.
- Signs
- Usually none. Sometimes flu-like aches and swollen glands. Eye infection can cause blurry vision.
- How it's found
- Blood antibody test (shows if you've ever been infected).
- How to stop it
- Cook meat to safe temperatures. Wash hands after gardening, raw meat and litter boxes. Pregnant? Let someone else scoop the litter.
Giardia
Giardia duodenalis
The "beaver fever" bug from streams, wells and daycare hands.
Full file
- How common
- One of the most common gut parasites in the U.S. (CDC).
- What it does
- Sticks to the small intestine wall with a suction disc and blocks the absorption of fats and nutrients.
- Nerves & brain
- Not a nerve invader, but long infections drain nutrients and energy, and can leave lasting gut sensitivity.
- Signs
- Greasy, floating stools, gas, cramps, bloating, nausea, weight loss. Can trigger lactose intolerance for a while after.
- How it's found
- Stool antigen test or a PCR stool panel.
- How to stop it
- Filter or boil water from lakes, streams and untested wells. Wash hands after diapers and bathroom.
ProtozoaWaterPets & animalsPeople
Cryptosporidium
Cryptosporidium spp.
Survives chlorine for days. Pools are a top source.
Full file
- How common
- A leading cause of waterborne outbreaks in the U.S., often from pools and splash pads.
- What it does
- Infects the lining of the intestine, causing heavy watery diarrhea. Severe in people with weak immune systems.
- Nerves & brain
- No direct nerve effects. Dehydration from heavy diarrhea can cause weakness and dizziness.
- Signs
- Watery diarrhea for 1–2 weeks, cramps, nausea, low fever.
- How it's found
- Specific stool test. Routine stool exams can miss it, so ask for it by name.
- How to stop it
- Don't swallow pool water. Stay out of pools for 2 weeks after diarrhea. Wash hands after farm animals, especially calves.
Cyclospora
Cyclospora cayetanensis
Rides in on berries, lettuce, basil and cilantro.
Full file
- How common
- Causes summer outbreaks in the U.S. nearly every year, traced to fresh produce.
- What it does
- Infects the small intestine. Without treatment, symptoms can come and go for a month or more.
- Nerves & brain
- No direct nerve effects. Long bouts cause heavy fatigue.
- Signs
- Watery diarrhea, loss of appetite, bloating, gas, cramps, fatigue.
- How it's found
- Special stool test or PCR panel. Ask for it specifically.
- How to stop it
- Wash produce well. Washing helps but may not remove it completely, so watch outbreak alerts.
Entamoeba
Entamoeba histolytica
An amoeba that can eat through the gut wall.
Full file
- How common
- Mostly in places with poor sanitation. Travelers can bring it home.
- What it does
- Lives in the colon. It can invade the gut wall causing bloody dysentery, and travel to the liver to form abscesses.
- Nerves & brain
- Rarely reaches the brain.
- Signs
- Belly pain, bloody diarrhea, fever. Liver abscess causes pain under the right ribs.
- How it's found
- Stool antigen or PCR test. Blood test for liver infection.
- How to stop it
- Safe water and food when traveling: bottled or boiled water, cooked food, peeled fruit.
Pinworm
Enterobius vermicularis
Tiny white worms that come out at night to lay eggs.
Full file
- How common
- The most common worm infection in the U.S., especially in kids.
- What it does
- Lives in the colon. Females crawl out at night to lay eggs around the anus. Eggs spread to hands, sheets and toys.
- Nerves & brain
- The night itching disrupts sleep, which can show up as irritability and trouble focusing.
- Signs
- Itching around the anus, worse at night. Restless sleep.
- How it's found
- Tape test first thing in the morning. Stool tests usually miss it.
- How to stop it
- Wash hands, trim nails, wash bedding in hot water. Treat the whole household at once.
Giant roundworm
Ascaris lumbricoides
Can grow over 30 cm (a foot) long.
Full file
- How common
- Infects hundreds of millions of people worldwide (WHO).
- What it does
- Eggs swallowed from soil or produce hatch, travel through the lungs, are coughed up and swallowed, then grow in the intestine. Heavy loads can block the bowel.
- Nerves & brain
- No direct nerve effects. In kids, heavy loads hurt growth and learning through malnutrition.
- Signs
- Often none. Cough while larvae pass the lungs, belly pain, worms in stool.
- How it's found
- Stool exam for eggs.
- How to stop it
- Wash and peel produce, wash hands after soil, avoid produce grown with untreated manure.
Hookworm
Necator & Ancylostoma
Burrows through bare feet and drinks your blood.
Full file
- How common
- Hundreds of millions worldwide. Once common in the U.S. South and still found in some poor rural areas.
- What it does
- Larvae enter through the skin, travel to the gut and latch on with hooked mouths to feed on blood.
- Nerves & brain
- Blood loss causes iron-deficiency anemia: fatigue, brain fog and, in kids, slower learning.
- Signs
- Itchy rash where it entered, then fatigue, pale skin, shortness of breath.
- How it's found
- Stool exam. Blood count shows anemia and often high eosinophils.
- How to stop it
- Wear shoes on soil and sand, especially where sanitation is poor.
Threadworm
Strongyloides stercoralis
Can live in you for decades by reinfecting you from the inside.
Full file
- How common
- Tens of millions worldwide. Found in the southeastern U.S. and Appalachia.
- What it does
- Enters through skin like hookworm, but can complete its life cycle inside you, so an infection can last a lifetime.
- Nerves & brain
- If the immune system is suppressed (steroids, some cancers), it can spread through the body and carry gut bacteria to the brain, causing meningitis.
- Signs
- Itchy moving rash, belly pain, diarrhea, cough. Often none for years.
- How it's found
- Blood antibody test is best. Stool tests often miss it.
- How to stop it
- Wear shoes on soil. Anyone from a high-risk area should get tested before starting steroid medicines.
Pork tapeworm
Taenia solium
The tapeworm that can end up in your brain.
Full file
- How common
- Common in parts of Latin America, Africa and Asia. The WHO calls it a leading cause of preventable epilepsy.
- What it does
- Eating undercooked pork with cysts gives you an intestinal tapeworm. Swallowing its eggs (from a carrier's unwashed hands) lets larvae form cysts in muscle, eyes and brain.
- Nerves & brain
- Brain cysts (neurocysticercosis) cause seizures, headaches, confusion and pressure on the brain.
- Signs
- Gut worm: mild belly upset, segments in stool. Brain cysts: seizures, severe headaches.
- How it's found
- Stool test for the gut worm. Blood test plus CT or MRI scan for cysts.
- How to stop it
- Cook pork to 145°F (63°C). Wash hands after the bathroom and before handling food.
Beef tapeworm
Taenia saginata
Can reach 4–10 meters long in the gut.
Full file
- How common
- Found worldwide wherever raw or rare beef is eaten.
- What it does
- Undercooked beef with cysts grows into a long tapeworm in the small intestine. Segments break off and pass in stool.
- Nerves & brain
- Unlike the pork tapeworm, it doesn't form brain cysts.
- Signs
- Often mild: belly discomfort, appetite changes, moving segments in stool.
- How it's found
- Stool exam or spotting segments.
- How to stop it
- Cook beef to 145°F (63°C) and rest 3 minutes. Skip raw beef dishes.
Fish tapeworm
Dibothriocephalus latus
The longest human tapeworm, over 10 meters.
Full file
- How common
- Found where raw freshwater fish or wild salmon is eaten, including North America.
- What it does
- Raw or undercooked fish passes larvae that grow in the intestine and absorb vitamin B12.
- Nerves & brain
- B12 loss can damage nerves and their myelin insulation: numbness, tingling, balance and memory problems.
- Signs
- Often none. Fatigue, belly discomfort, B12-deficiency anemia.
- How it's found
- Stool exam for eggs. Blood test for B12.
- How to stop it
- Cook fish to 145°F (63°C), or freeze it properly before eating raw.
Trichinella
Trichinella spiralis & others
Larvae coil up inside your muscle cells.
Full file
- How common
- Rare in U.S. commercial pork today. Bear and other wild game are now the main U.S. sources.
- What it does
- Larvae from undercooked meat mature in the gut, and their young travel through the blood to settle in muscle.
- Nerves & brain
- Heavy infections can inflame the heart and brain (encephalitis).
- Signs
- Belly upset first, then muscle pain, fever, swelling around the eyes.
- How it's found
- Blood antibody test and blood count. Sometimes a muscle biopsy.
- How to stop it
- Cook pork to 145°F (63°C) and wild game to 160°F (71°C). Freezing doesn't reliably kill it in wild game.
Anisakis
Anisakis simplex
Burrows into your stomach wall hours after sushi.
Full file
- How common
- Common in raw saltwater fish dishes. Most cases are reported where raw fish is eaten often.
- What it does
- Larvae in raw fish try to burrow into the stomach or intestine wall. They can't survive long in people, but the damage and allergy can be intense.
- Nerves & brain
- No direct nerve effects. Can trigger severe allergic reactions.
- Signs
- Sudden sharp stomach pain, nausea and vomiting a few hours after eating. Hives or anaphylaxis in allergic people.
- How it's found
- Usually found and removed by endoscopy (a camera down the throat).
- How to stop it
- Cook fish to 145°F (63°C), or eat raw fish that was frozen to −4°F (−20°C) for 7 days.
Dog & cat roundworm
Toxocara canis & cati
Pet worms that wander into children's eyes.
Full file
- How common
- Blood studies show millions of Americans have been exposed. The CDC lists it as a neglected parasitic infection.
- What it does
- Eggs in soil and sandboxes contaminated by pet waste hatch in people, and the larvae wander through organs instead of maturing.
- Nerves & brain
- Larvae can reach the eye and cause permanent vision loss. Rarely, the brain.
- Signs
- Often none. Fever, cough, wheezing, belly pain, or a vision problem in one eye.
- How it's found
- Blood antibody test. Eye exam.
- How to stop it
- Deworm pets regularly, pick up their waste, cover sandboxes, wash hands after playing outside.
Rat lungworm
Angiostrongylus cantonensis
Goes straight for the brain.
Full file
- How common
- Found in Hawaii and the U.S. Gulf Coast, and spreading.
- What it does
- Swallowed larvae from raw snails, slugs or produce they crawled on travel to the brain and its lining.
- Nerves & brain
- Causes eosinophilic meningitis: severe headache, stiff neck, burning or tingling skin pain, sometimes paralysis.
- Signs
- Severe headache, stiff neck, tingling or painful skin, nausea.
- How it's found
- Spinal fluid test showing high eosinophils. PCR in specialized labs.
- How to stop it
- Never eat raw snails or slugs. Wash produce well, especially in Hawaii and the Gulf South.
Scabies mite
Sarcoptes scabiei
Burrows tunnels in your skin.
Full file
- How common
- Over 200 million people worldwide at any time (WHO).
- What it does
- Tiny mites dig tunnels in the top layer of skin to lay eggs. Spread by long skin contact and shared bedding.
- Nerves & brain
- The intense itch, worse at night, wrecks sleep.
- Signs
- Intense itching and a pimple-like rash, often between fingers, wrists, waist and groin.
- How it's found
- Skin exam, sometimes a skin scraping under a microscope.
- How to stop it
- Treat close contacts at the same time. Wash bedding and clothes in hot water.