Most people picture gut parasites coming from somewhere tropical. The evidence tells a different story, the routes that actually cause infections in countries like the UK are far more ordinary, and most of them run through water and other people.
When researchers at the University of Glasgow and Public Health Scotland reviewed 75 giardiasis outbreak investigations across high-income countries, the two most common routes were contaminated drinking water and contact with children of nappy-wearing age. Food outbreaks usually traced back to an infected food handler rather than the food itself. Travel multiplies your exposure to all of these rather than being a separate cause, and pets matter less than most people assume. Knowing the routes is worth your time, because research on a UK farm outbreak found that simply being aware of the risk was protective.
How do gut parasites spread?
Almost every gut parasite travels the same way, known as the faecal-oral route. Microscopic eggs or cysts leave the body in an infected person's or animal's faeces, survive in water, on food, on hands or on surfaces, and infect the next person who swallows them.
That mechanism explains why the biggest risks are so everyday. A pool swim, a nursery pickup or a shared towel can be enough, no jungle trek required. The same Glasgow review made the point that giardiasis in the UK is often wrongly assumed to be a travel illness, which is one reason infections picked up at home go under-reported.
Our guide to the main gut parasites covers the organisms themselves and their symptoms. This page is about how they reach you, and how to stop them.
Why is water the biggest route?
Water is the standout. A review that counted every reported waterborne parasite outbreak worldwide between 2017 and 2022 found 416 of them. Cryptosporidium caused 77%, and 92% of those Cryptosporidium outbreaks were linked to swimming pools or other recreational water.
Pools are vulnerable because Cryptosporidium has a tough, egg-like outer shell that survives normal chlorine levels. A UK review co-authored by the national Cryptosporidium Reference Unit explains that disinfection cannot clear it once it is in the water, and pool filters remove it only imperfectly.
The practical rules follow directly. Try not to swallow pool water, keep anyone with diarrhoea out of the pool, and if you have had a confirmed infection, stay out of pools for two weeks after your symptoms stop. That is standing NHS advice, because you can keep shedding the parasite after you feel better.
How do parasites pass from person to person?
Giardia and Cryptosporidium spread readily wherever nappies are changed and small hands are washed imperfectly: households, nurseries, playgroups. In the outbreak review above, contact with children of nappy-wearing age sat alongside drinking water as the most common route to infection.
This is also why the NHS sometimes looks beyond the person who is ill. For giardiasis, NHS guidance notes that the people you live with may also need to be tested and treated, because household members can carry and pass on the parasite without any symptoms.
Threadworms, the most common worm infection in UK children, take this to its logical end. The NHS pathway usually treats the whole household at once, because the eggs pass so easily between family members.
Can you catch a gut parasite from food?
Yes, though less often than from water, and usually via a person. In the Glasgow review, eight of the ten food-linked giardiasis outbreaks traced back to an infected food handler rather than to contaminated ingredients.
The food route is still bigger than the outbreak record suggests. Giardia alone is estimated to cause around 28 million cases of diarrhoea a year worldwide through contaminated food, and researchers believe most foodborne outbreaks are never detected, because finding parasite cysts in food is technically difficult.
Fresh produce deserves the most respect: fruit, salad leaves and herbs eaten raw, sometimes washed in contaminated water long before they reach you. Imported produce has repeatedly carried Cyclospora into the UK. Washing produce under clean running water, cooking food through and washing your hands before preparing meals removes most of the risk.
Why does travel raise the risk so much?
Diarrhoea affects roughly 40-60% of international travellers. Most of it is bacterial, and most of it settles within a few days.
Parasites behave differently. They are the leading cause among travellers whose symptoms persist after they get home. A clinical review in JAMA puts the threshold at 14 days: once diarrhoea lasts that long, parasites such as Giardia and Cryptosporidium move to the top of the suspect list. Around 3% of travellers to lower-income regions develop persistent diarrhoea of this kind.
Abroad, the familiar advice holds. Drink bottled, boiled or filtered water where tap water is not safe, be wary of ice and raw salads in higher-risk places, and check the food and water advice for your destination on TravelHealthPro, the UK's official travel health service.
If your gut has not settled within a couple of weeks of coming home, see your GP and tell them where you travelled. Testing can then help identify which organism is involved: an at-home parasite stool test screens for Giardia, Cryptosporidium and other common culprits, and it works alongside your GP's assessment, never instead of it.
Can you catch a gut parasite from your pet?
Occasionally, but far less often than older advice suggested. Studies using modern genetic typing have found that the Giardia strains infecting people mostly come from other people and from water, while pets usually carry animal-adapted strains that rarely move to humans.
The picture for Cryptosporidium in dogs and cats is the same. They mostly carry their own dog and cat species of the parasite, and although confirmed cases of pet-to-owner spread exist, they appear to be uncommon.
The honest summary is mostly people and water, occasionally animals. Wash your hands after handling pets and especially their waste, and keep up routine worming, but the family dog is not the main risk in your house.
What about farm visits and petting zoos?
Farm animals, especially calves and lambs, are a genuine route. UKHSA investigated a large 2023 outbreak at a lamb-petting event in south-west England: 23 laboratory-confirmed Cryptosporidium cases, and 83 probable cases once attendees were surveyed, which shows how much illness never reaches the official count. Going into the petting pen roughly doubled the odds of becoming ill.
The most useful finding was about awareness. People who already knew that farm animals can pass on stomach bugs were markedly less likely to become ill. Knowing the risk changes behaviour enough to prevent infection.
GOV.UK's farm visit advice is specific on the method: wash hands with soap and water after touching animals, fences or pens and before eating. Hand gels do not reliably kill these organisms. Soap and water does.
What actually prevents gut parasites?
Prevention here is unglamorous, and it maps directly onto the routes above.
- Wash hands with soap and water after the toilet, after nappy changes, after touching animals, and before preparing or eating food. This one habit interrupts nearly every route on this page.
- At the pool, try not to swallow water, keep anyone with diarrhoea out, and stay out yourself for two weeks after any diarrhoeal illness.
- When travelling, take care with water, ice and raw produce in higher-risk destinations, and check TravelHealthPro before you go.
- On farms and at petting events, use soap and water rather than gel, eat well away from animal pens, and supervise children's handwashing closely.
- If someone at home is infected, follow the NHS hygiene advice: frequent handwashing, hot-wash potentially contaminated bedding and clothing, keep towels separate, and stay off school or work until 48 hours after symptoms stop.
When should you see a GP?
The NHS advises asking for an urgent GP appointment, or calling 111, if you have had diarrhoea for more than seven days or there is blood in your stool. Tell them about any recent travel. See your GP as well if you have unexplained weight loss, signs of dehydration, persistent vomiting, or a change in bowel habit that lasts more than three weeks, and seek advice early if your immune system is weakened by illness or treatment, because cryptosporidiosis in particular can become serious.
A stool test can identify a parasite, but it is never an alternative to seeing a doctor about symptoms. If any of the above applies to you, the GP comes first, whatever a test does or does not find. This guide is for information only and does not replace medical advice.
Sources
- Krumrie S, et al. (2022). A scoping review of risk factors and transmission routes associated with human giardiasis outbreaks in high-income settings. Current Research in Parasitology and Vector-Borne Diseases. PubMed | DOI
- Bourli P, et al. (2023). Waterborne transmission of protozoan parasites: a review of worldwide outbreaks, an update 2017-2022. Journal of Water and Health. PubMed | DOI
- Wood M, et al. (2019). Role of filtration in managing the risk from Cryptosporidium in commercial swimming pools: a review. Journal of Water and Health. PubMed | DOI
- DuPont HL (2016). Persistent diarrhea: a clinical review. JAMA. PubMed | DOI
- Butler D, et al. (2022). Travel-related gastrointestinal diseases: assessment and management. Public Health Challenges. PubMed | DOI
- Ryan U, et al. (2018). Giardia: an under-reported foodborne parasite. International Journal for Parasitology. PubMed | DOI
- Cai W, et al. (2021). Zoonotic giardiasis: an update. Parasitology Research. PubMed | DOI
- Li J, et al. (2021). Advances in molecular epidemiology of cryptosporidiosis in dogs and cats. International Journal for Parasitology. PubMed | DOI
- Peake L, et al. (2024). A large cryptosporidiosis outbreak associated with an animal contact event in England: a retrospective cohort study, 2023. Epidemiology and Infection. PubMed | DOI
- NHS. Giardiasis. nhs.uk/conditions/giardiasis
- NHS. Threadworms. nhs.uk/conditions/threadworms
- UKHSA. Avoiding infection on farm visits: advice for the public. GOV.UK
- UKHSA. Cryptosporidium: public advice. GOV.UK
- TravelHealthPro (NaTHNaC). Destination-specific food and water advice. travelhealthpro.org.uk
