Most people think parasites are something that only happens in faraway countries with dirty water. But if you’ve ever had unexplained diarrhea that won’t go away, or your child won’t stop scratching their bottom at night, you’ve likely dealt with one of the most common parasitic infections in the U.S. - giardia or pinworms. These aren’t rare oddities. They’re everyday problems that affect millions, especially kids, travelers, and families with young children.

What Giardia Does to Your Body

Giardia is a tiny, single-celled parasite called Giardia lamblia. It doesn’t just give you an upset stomach - it attacks the lining of your small intestine. Think of your gut like a sponge that absorbs nutrients. Giardia flattens the tiny finger-like projections (microvilli) that do the absorbing. That’s why people with giardia lose weight, feel tired, and get bloated even when they’re eating normally.

You get it by swallowing something contaminated. Not just dirty water - though that’s the biggest source. It can be from swimming in a lake, drinking from a stream, or even eating food washed with untreated water. The parasite survives as a hard shell called a cyst. These cysts can live for months in cold water. Chlorine in pools won’t kill them. Boiling water for just one minute does. So does using a filter with pores smaller than 1 micron.

Symptoms usually show up 1 to 2 weeks after exposure. Diarrhea is watery and smells awful. You’ll have gas, cramps, nausea, and sometimes vomiting. Fatigue hits hard. Some people lose 10 pounds in a few weeks. In kids, it can slow growth. The scary part? About 1 in 3 people don’t feel sick at all but still spread the parasite.

Pinworms: The Itch That Won’t Quit

Pinworms are small, white worms about the length of a staple. They live in your colon and rectum. At night, the female worm crawls out through the anus to lay eggs on the skin around your bottom. That’s when the itching starts - intense, burning, and worse when you’re lying down.

It’s not just kids. Anyone can get pinworms, but 75% of cases happen in households with children under 10. Why? Kids touch everything - toys, desks, doorknobs - and then put their hands in their mouths. Eggs stick to fingernails, bedding, clothes, even dust. You can inhale them. You can swallow them. Once inside, they hatch in 2 to 6 weeks and start the cycle again.

Most people don’t realize they have pinworms until the itching starts. Sometimes you’ll see the worms - they’re white, move slowly, and look like threads. But often, there’s no visible sign. That’s why the “tape test” is the gold standard: press clear tape against the skin around the anus first thing in the morning, then stick it to a slide. Under a microscope, you’ll see eggs. One test catches half the cases. Three tests catch 9 out of 10.

How Doctors Diagnose These Infections

For giardia, most clinics now use a stool antigen test. It looks for parasite proteins, not the bug itself. It’s 95% accurate. Old-school microscopy - looking under a microscope for cysts - misses up to 30% of cases. If you’ve had diarrhea for more than a week and your doctor doesn’t test for giardia, ask for it.

For pinworms, the tape test is simple and cheap. You can do it at home. Stick the tape on the skin around the anus before you shower or use the bathroom. Bring it to your doctor. No need for a stool sample. Many parents miss this step and end up treating the wrong thing.

Doctors in primary care miss giardia in about 1 out of 3 cases, according to a 2022 study. That’s partly because symptoms mimic food poisoning or a virus. If you’ve been to the doctor twice and nothing’s helping, get tested for parasites.

Treatment: What Actually Works

Good news: both infections are easy to treat. Bad news: if you don’t treat everyone in the house, they come back.

For giardia, the most common drug is metronidazole. You take 250 mg three times a day for 5 to 7 days. It works in 80 to 95% of cases. But side effects? A metallic taste (78% of people report it), nausea, and sometimes dizziness. Tinidazole is a one-dose alternative - 2 grams in a single pill. Fewer side effects, same cure rate. Nitazoxanide is another option, especially for kids as young as 1 year. It’s taken twice a day for 3 days.

For pinworms, the go-to is mebendazole: one 100 mg tablet, then another two weeks later. Albendazole (400 mg single dose) works just as well. Pyrantel pamoate is also used, especially in kids. The CDC updated its guidelines in January 2024 to recommend triple-dose albendazole (400 mg on days 1, 8, and 15) for stubborn cases. That works in 98% of cases.

Here’s what no one tells you: medication alone isn’t enough. If you don’t clean your environment, you’ll get reinfected. That’s why 2 out of 3 families report reinfection within 3 months.

Stopping the Spread: The Real Secret

Medication kills the worms. Cleaning kills the eggs.

For giardia: wash your hands with soap and water after every bathroom visit, before eating, and after changing diapers. Don’t share towels. Avoid swimming in public pools for at least 2 weeks after symptoms stop. If you work in food service or childcare, stay home until you’re symptom-free for 48 hours.

For pinworms: this is critical. Wash all bedding, pajamas, and underwear in hot water on the same day you start treatment. Dry them on high heat. Vacuum carpets and wipe down toys, doorknobs, and countertops. Cut fingernails short. Don’t let kids scratch their bottoms - it spreads eggs to their hands. Everyone in the house needs treatment, even if they feel fine. The CDC says 75% of household members are infected when one person has it.

One parent on Reddit shared that after treating her son twice and still seeing worms, she hired a professional cleaner to deep-clean the entire house. The third round of treatment worked. No more itching.

Who’s at Highest Risk?

Children under 10. People who travel to developing countries. Campers and hikers who drink from streams. Parents and caregivers of young kids. People in daycare centers or nursing homes. Immune-compromised individuals - like those with HIV - can get chronic giardia that lasts months or longer.

Even in the U.S., giardia causes over 1 million cases a year. Pinworms affect 40 to 80 million Americans. That’s not a small number. It’s more than the flu in some years.

Why This Keeps Happening

Giardia is tough. It survives chlorine. It doesn’t care if the water looks clean. In places with old pipes or poor sanitation, it’s everywhere. Even in Seattle, outbreaks happen after heavy rains wash animal waste into reservoirs.

Pinworms spread because we don’t treat them like the infections they are. We think, “It’s just itching - it’ll go away.” But it won’t. And it won’t stay just with your child. It’ll be on your couch, your phone, your toothbrush.

Drug resistance is starting to show up. In Southeast Asia, 15% of giardia cases don’t respond to metronidazole. In the U.S., it’s still around 5%. But that number is rising. Researchers are testing a giardia vaccine - early results show promise. But for now, prevention is still the best medicine.

What to Do Next

If you or your child has:

• Diarrhea lasting more than a week, especially after travel or swimming
• Unexplained weight loss or fatigue with stomach issues
• Intense nighttime itching around the anus

Don’t wait. See your doctor. Ask for a giardia stool test or a tape test for pinworms. Start treatment the same day. Treat everyone in the house. Wash everything. Clean surfaces. Change bedding. Repeat the treatment after two weeks.

It’s not glamorous. It’s not exciting. But it works. One family in Minnesota followed all the steps - treatment, laundry, cleaning, repeat dose - and their 5-year-old hasn’t had a single itch in 14 months.

Parasites don’t care about your income, your education, or your hygiene habits. They care about one thing: a way in. Stop the way in. Break the cycle. You don’t need fancy tools. Just soap, hot water, and a little discipline.