A microscopic parasite uses a clever shell to survive in water for months.

Nature · Published 3 October 2026 Swipe more like this →

In short: Giardia is a resilient parasite that turns your small intestine into a feeding ground. By encasing itself in a protective shell, it survives outside the body in soil or water, waiting to be swallowed. Once inside, it sheds its armor to multiply, often leaving you with persistent, uncomfortable digestive symptoms.

Giardia duodenalis is a microscopic parasite that thrives by attaching to the lining of the small intestine. Its lifecycle is defined by two distinct forms: the motile trophozoite, which absorbs nutrients from the host, and the environmentally resistant cyst. When the parasite moves toward the colon, it forms a hard outer shell, allowing it to survive for weeks or even months in external environments like lakes, streams, or soil. This shell is notably resistant to common disinfectants, including chlorine bleach, making it a persistent threat in water supplies.

Infection occurs through the fecal-oral route, often requiring the ingestion of only a few cysts. Once swallowed, these cysts break down in the gut, releasing trophozoites that multiply via binary fission. While many infections resolve within two to six weeks, the parasite can cause significant distress, including fatty, foul-smelling diarrhea, bloating, and abdominal cramps. Some individuals remain asymptomatic carriers, unknowingly shedding the parasite in their stool and contributing to its spread through person-to-person contact, contaminated food, or tainted water.

Children are particularly susceptible, especially in childcare settings where contact with contaminated surfaces or diapers is frequent. While the parasite is found globally, it is a common cause of waterborne disease in the United States, often appearing in public water systems, swimming pools, and private wells. Treatment typically involves antibiotics, though prevention remains the most effective strategy. Proper handwashing, avoiding untreated water, and maintaining sanitation are essential, as the parasite can persist in the environment long after an infected person has recovered.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• The microscopic parasite that turns your drinking water into a digestive nightmare
• The microscopic parasite that turns water into a biological hazard
• A microscopic parasite with a smiley face is the world's most common intestinal invader.
• The microscopic passenger that has been hitchhiking since 1681
• Parasites are not just distant threats; they are complex organisms living off our biology.
• A desert lizard's venom peptide became modern diabetes drugs
• The world's largest ray can be told apart by a T on its back
• A common genetic quirk that turns your skin yellow might actually protect your heart.
• The longest bony fish in the sea may explain old sea serpent sightings
• Every gilt-head bream starts life male, and some later become female
• A Philippine fruit bat has the longest forearms of any bat alive
• Medieval English shoppers paid a whole sheep's worth for one pound of ginger

More in Organisms · More in Nature · Latest · A–Z · All topics