What is Cryptosporidium?
Cryptosporidium is a microscopic parasite that infects the intestines and causes cryptosporidiosis, a diarrhoeal illness. You may also see it written as Cryptosporidium spp., where "spp." simply means the name covers several species of the parasite rather than one.
It spreads through contaminated water and food, and through contact with infected people and animals. It is one of the leading causes of diarrhoeal illness worldwide, and because it can survive in properly chlorinated water, it lies behind a large share of waterborne gut infections, including in the UK.
Cryptosporidiosis affects people of all ages, but it is most prominent in children aged 1-5 and in travellers to regions where clean water is harder to come by.
How does Cryptosporidium spread?
Cryptosporidium can survive in water, food, soil and on surfaces contaminated with infected faeces. Infection happens when you swallow the parasite’s oocysts: tough, egg-like capsules that protect it outside the body. Once swallowed, the oocysts 'hatch' in the intestines, releasing the active form of the parasite and starting the infection.
Contaminated water
Swimming pools, hot tubs, lakes and rivers can become contaminated if someone with the infection has diarrhoea in the water, or carries traces of faeces in with them. Water is this parasite’s natural territory. When researchers counted every reported waterborne parasite outbreak worldwide between 2017 and 2022, 416 in total, 77% were caused by Cryptosporidium, and 92% of those Cryptosporidium outbreaks were linked to swimming pools or other recreational water.
Outbreaks from drinking water also happen, mainly where water treatment is insufficient, which is why the risk is higher in countries with limited water and sanitation infrastructure.
Why doesn’t chlorine kill Cryptosporidium?
Chlorine deals with most of the bugs people bring into a pool, but not this one. The oocyst’s thick outer wall lets it survive for days at normal pool chlorine levels, so pools depend on their filters to physically strain the parasite out. A review co-authored by the UK’s Cryptosporidium Reference Unit explains that this filtering is imperfect: oocysts are only a few thousandths of a millimetre across, and some pass through. A 2026 review of pool-linked outbreaks reported between 2020 and 2025 found Cryptosporidium was still one of the two most common causes, with most outbreaks traced to failures in disinfection and maintenance.
Contaminated food
Fresh produce can carry the parasite, particularly if it was washed in contaminated water or handled by someone with the infection. Cooking food thoroughly, to above 75°C, kills it.
Person-to-person contact
The parasite spreads easily wherever people are in close contact: nurseries, care homes and households. It can also be passed on during sex, particularly oral-anal sex.
Someone who has had cryptosporidiosis can continue to shed the parasite in their faeces for several weeks after their symptoms stop, which is why hygiene still matters once you feel better.
Animal contact
Farm animals, particularly young calves, lambs and goats, can carry Cryptosporidium and pass it to people. Petting farms are a recurring source of UK outbreaks: between January and May 2013, around 130 individuals were affected across twelve outbreaks in England.
A more recent outbreak shows how it happens. In 2023, the UK Health Security Agency investigated a large outbreak at a lamb-petting event in south-west England. A laboratory confirmed 23 cases, but when attendees were surveyed, researchers identified 83 probable cases, far more than the official count, which shows how much of this illness goes unrecorded. Going into the petting pen roughly doubled the odds of becoming ill compared with watching from outside. The same attraction went on to record four outbreaks between 2023 and 2025, 39 confirmed cases in all, despite complying with industry hygiene standards.
One finding from the 2023 investigation is worth holding on to: visitors who already knew farm animals can pass on gut infections were much less likely to become ill. Knowing the risk, and washing your hands properly because of it, is genuinely protective.
Is Cryptosporidium common in the UK?
Yes, and 2023 showed how common it can get. In August and September that year, laboratories in England and Wales recorded their largest ever increase in Cryptosporidium cases. UKHSA and Public Health Wales investigated by comparing 203 people who had cryptosporidiosis with 614 people who had a different gut infection, to see what set the Cryptosporidium group apart.
People with cryptosporidiosis were around five times more likely to have used a swimming pool, and around six times more likely to have travelled to Spain, than the comparison group. Children under 5 were the age group most strongly affected. No single cause explained the surge; swimming pools and foreign travel each played a part.
For UK families the practical reading is simple: the riskiest combination is young children and swimming pools, whether at home or on holiday.
What symptoms does Cryptosporidium cause?
Symptoms usually appear 2 to 10 days after swallowing the parasite, and they vary from person to person. Some people have no symptoms at all, but can still pass the parasite on.
Common symptoms:
- Watery diarrhoea
- Stomach cramps and pain
- Nausea and vomiting
- A mild fever
- Loss of appetite and weight loss
- Dehydration, from the fluid lost to diarrhoea
In otherwise healthy people the illness usually lasts around two weeks, though it can go on longer. It also has a habit of appearing to clear and then returning a few days later before you fully recover; UKHSA’s public advice describes exactly this stop-start pattern, so do not assume a good day means the infection has gone.
Can cryptosporidiosis be serious?
For most people it is unpleasant but clears up on its own. For people with weakened immune systems it can be a different illness altogether. A 2024 review of 134 studies found that in people with HIV, people having chemotherapy and organ transplant recipients, the infection can become chronic and severe, and the standard drug treatment works poorly. The major Lancet review of the field reached the same conclusion: treatment helps in healthy people and has no proven effect in people with advanced HIV.
If your immune system is weakened by a condition or a treatment and you develop watery diarrhoea, contact your GP promptly. That situation calls for medical care, not a home test.
How is cryptosporidiosis diagnosed?
Cryptosporidiosis is confirmed by testing a stool sample, which is checked for the parasite’s oocysts or its DNA. Its symptoms look like many other tummy bugs, so a sample is the only way to know this particular parasite is the cause. A test identifies what is present; the diagnosis itself comes from a clinician reading the result alongside your symptoms.
Oocysts are not shed evenly, so a single sample can miss the infection. Doctors may ask for more than one sample over a few days to get a reliable answer.
NHS testing is free: your GP can send a stool sample to an NHS laboratory. Tell them about any recent travel abroad, and any pool, lake or farm visits, because it helps the laboratory decide what to look for.
How can you test for Cryptosporidium at home?
Several methods can detect parasites such as Cryptosporidium, including examining a sample under a microscope, antigen testing and DNA-based testing. Antigen and DNA-based methods are generally more sensitive than traditional microscopy, a point the Lancet review above also makes.
Our gut parasite test identifies 28 different types of parasites, pathogenic bacteria and viruses from a stool sample you collect at home.
Common intestinal parasites such as Cyclospora, Giardia Lamblia and Cryptosporidium can cause many of the same symptoms, which is why testing usually checks several parasites at once. A home test can identify which parasite is present, but it does not diagnose a condition, and it is never a substitute for seeing your GP, especially if you have any of the warning signs listed further down this page.
How is cryptosporidiosis treated?
There is no specific treatment for cryptosporidiosis. UKHSA advises that most people with a healthy immune system recover within about a month, and care in the meantime focuses on the symptoms, above all replacing the fluid lost to diarrhoea.
While you recover:
- Drink plenty of fluids; a pharmacist can advise on rehydration
- Get plenty of rest, particularly while symptoms last
- Eat small, frequent meals
- Foods high in fibre can help to firm up loose stools
You remain infectious while unwell, so stay off work or school until you have been free of diarrhoea and vomiting for 48 hours, and do not prepare food for anyone else until then. If your immune system is weakened, do not wait the illness out: speak to your GP, because treatment in that situation needs specialist input.
Can Cryptosporidium be prevented?
You cannot remove every chance of meeting this parasite, but a few habits cut the risk substantially. The petting-farm research above found that simply knowing the risks made visitors measurably less likely to fall ill, so the habits below are worth actually remembering, not just reading.
Water safety
Avoid swallowing water when swimming in pools, lakes or rivers. When travelling in countries where the tap water may be unsafe, drink bottled water and avoid ice in drinks.
If you or your child have had cryptosporidiosis, UKHSA advises staying out of swimming pools until 2 weeks after the diarrhoea has stopped. The parasite is still shed after symptoms end, chlorine does not kill it, and one recovering swimmer can contaminate an entire pool.
Food hygiene
Wash or peel fruit and vegetables before eating them raw. Cook meat thoroughly, since proper cooking kills the parasite, and wash your hands before preparing or handling food.
Farm and animal contact
Wash your hands with soap and warm water after farm visits, petting zoos and handling animals, and help young children wash theirs properly. Alcohol hand gel is not a substitute here: public health advice is clear that gels and wipes do not work well against the organisms found on farms, and Cryptosporidium’s tough outer shell makes it a prime example. On farm visits, eat and drink only in the designated areas.
Around the home
If someone in your household is ill, wash their clothes, bedding and towels on the hottest cycle, make sure everyone uses their own towel, and clean toilet seats, flush handles and taps with detergent and hot water followed by disinfectant. Keep this up until at least 48 hours after symptoms stop.
When should you see a GP?
Most cryptosporidiosis clears up without medical help. See a GP if you have:
- Blood in your stool, or bleeding from your bottom
- Diarrhoea lasting more than a week
- Signs of dehydration you cannot put right by drinking, such as passing very little urine, dizziness or unusual drowsiness
- Severe stomach pain
- Unexplained weight loss
- A weakened immune system, from a condition or a treatment, alongside any of these symptoms: seek advice promptly rather than waiting
Mention any recent travel abroad and any pool, lake or farm exposure, because it changes what the laboratory tests for. Separately, any persistent change in your bowel habit lasting more than three weeks deserves a GP appointment in its own right, whatever the cause turns out to be.
This guide is for information only and does not replace medical advice. A home test can tell you what is in a sample; it cannot examine you, and it is never a substitute for seeing a GP about the warning signs above.
Sources
Studies referenced in this guide:
- Checkley W et al. (2015). A review of the global burden, novel diagnostics, therapeutics, and vaccine targets for cryptosporidium. The Lancet Infectious Diseases. https://pubmed.ncbi.nlm.nih.gov/25278220/
- Bourli P et al. (2023). Waterborne transmission of protozoan parasites: a review of worldwide outbreaks, an update 2017-2022. Journal of Water and Health. https://pubmed.ncbi.nlm.nih.gov/37902200/
- 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. https://pubmed.ncbi.nlm.nih.gov/31095512/
- Steinberg M et al. (2026). Swimming pool-associated infectious disease outbreaks from 2020 to 2025: an updated literature review. International Journal of Hygiene and Environmental Health. https://pubmed.ncbi.nlm.nih.gov/41861566/
- Williams SV et al. (2025). Retrospective case-case study investigation of a significant increase in Cryptosporidium spp. in England and Wales, August to September 2023. Eurosurveillance. https://pubmed.ncbi.nlm.nih.gov/40051396/
- 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. https://pubmed.ncbi.nlm.nih.gov/38800856/
- Sims B et al. (2026). Recurrent zoonotic Cryptosporidium transmission at an animal-contact visitor attraction: insights into infection prevention and control, South West England (2023-2025). Epidemiology and Infection. https://pubmed.ncbi.nlm.nih.gov/42522202/
- Balendran T et al. (2024). Cryptosporidiosis in a zoonotic gastrointestinal disorder perspective: present status, risk factors, pathophysiology, and treatment, particularly in immunocompromised patients. Journal of Tropical Medicine. https://pubmed.ncbi.nlm.nih.gov/39534184/
Public health guidance, rather than studies: UKHSA, Cryptosporidium: public advice, on GOV.UK (linked above; the source for the 2-week swimming rule, the 48-hour rules and the symptom duration), and Public Health England’s 2013 statement on illness linked to petting farms (linked above).
