# Cryptosporidium
**Cryptosporidium**, commonly called "Crypto", is a genus of microscopic intestinal protozoan parasites of the phylum [Apicomplexa](https://www.edgepedia.org/apicomplexa) that infect the epithelial cells of the gastrointestinal tract of humans and a wide range of vertebrates, causing the diarrheal disease cryptosporidiosis. It is the leading cause of waterborne disease in the United States.[1](https://www.cdc.gov/cryptosporidium/causes/index.html)
| Key fact | Detail |
|---|---|
| Classification | Genus of apicomplexan protozoa (family Cryptosporidiidae), established by Ernest Edward Tyzzer in 1907 with the type species *C. muris*[2](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=5806&mode=Info) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC321466/) |
| Infective stage | Thick-walled oocysts, rounded, 4.2 to 5.4 µm in diameter, each containing four sporozoites; infectious immediately upon excretion[4](https://www.cdc.gov/dpdx/cryptosporidiosis/) |
| Main human species | *C. parvum* (zoonotic) and *C. hominis* (anthroponotic) cause most human infections; 44 species are documented and 21 other species or genotypes have also been reported in humans[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) |
| Infectious dose | As few as 10 to 100 oocysts can cause infection in healthy volunteers[6](https://www.ncbi.nlm.nih.gov/books/NBK448085/) |
| Chlorine tolerance | Oocysts survive 3.5 to 10.6 days in water where free chlorine is maintained at CDC-recommended levels of 1 to 3 mg/L[7](https://www.cdc.gov/mmwr/pdf/ss/ss6403.pdf) |
| Landmark outbreak | The 1993 Milwaukee outbreak, the largest documented drinking water outbreak in US history, affected an estimated 403,000 people and caused over $96 million in combined healthcare costs and productivity losses[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) |
| Treatment | Nitazoxanide is the only FDA-approved drug, for immunocompetent patients at least one year old; no effective therapy exists for severe cryptosporidiosis in immunodeficient patients[8](https://www.cdc.gov/cryptosporidium/treatment/) • [5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) |
## Taxonomy and history
[Ernest Edward Tyzzer](https://www.edgepedia.org/ernest-edward-tyzzer), an American physician and parasitologist at Harvard Medical School, established the genus in 1907 when he described *Cryptosporidium muris* from the gastric glands of laboratory mice, and in 1912 he described *C. parvum*, a second species that infected the small intestine and had smaller oocysts.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC321466/) • [9](https://pmc.ncbi.nlm.nih.gov/articles/PMC7103119/) For roughly the next 50 years the parasite was little studied and was often confused with coccidian genera such as *Sarcocystis*, whose thin-walled oocysts resemble those of *Cryptosporidium*.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC321466/)
**Recognition as a pathogen came in stages.** In 1955 it was first identified as a disease-causing agent in diarrheic turkeys; in 1971 it was reported in association with diarrhea in young calves; and in 1976 the first two human cases of cryptosporidiosis were published.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC7103119/) In the 1980s the parasite gained wider recognition through fatal cryptosporidiosis in AIDS patients, zoonotic cases, waterborne outbreaks, and diarrheal disease in children.[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) Molecular tools, beginning with PCR-based genotyping, later clarified the taxonomy and validated multiple species in each vertebrate class.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC321466/) Palaeoparasitological work has identified *Cryptosporidium* in ancient Roman chamber pots from sites in Bulgaria, the earliest known evidence of human infection.[10](https://www.livescience.com/archaeology/romans/poop-encrusted-chamber-pots-from-the-roman-empire-reveal-oldest-known-human-cases-of-crypto-parasite)
## Biology and life cycle
The parasite completes its entire life cycle within a single host, reproducing both sexually and asexually.[6](https://www.ncbi.nlm.nih.gov/books/NBK448085/) [Infection](https://www.edgepedia.org/infection) begins when a host swallows sporulated thick-walled oocysts; each releases four sporozoites that attach to the apical surface of intestinal epithelial cells and invade to form an intracellular but extracytoplasmic parasitophorous vacuole, usually within the brush border.[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) • [4](https://www.cdc.gov/dpdx/cryptosporidiosis/) Within the vacuole the parasites undergo three rounds of asexual multiplication followed by a sexual phase producing male microgamonts and female macrogamonts.[4](https://www.cdc.gov/dpdx/cryptosporidiosis/) • [5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full)
Fertilization gives rise to two oocyst types. Thick-walled oocysts are environmentally resistant and leave the host in feces, already infectious, enabling immediate fecal-oral transmission; thin-walled oocysts rupture in the intestinal lumen and release sporozoites that autoinfect the same host, sustaining infection without re-exposure.[4](https://www.cdc.gov/dpdx/cryptosporidiosis/) • [5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) The oocyst wall is a durable protein-lipid-carbohydrate barrier that makes the parasite resistant to almost all conventional disinfection methods, including chlorination.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC3130844/) • [5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full)
## Transmission
Transmission is fecal-oral. Common routes include swallowing contaminated water from pools or lakes, contaminated drinks or ice, contaminated food, and hand-to-mouth contact after touching contaminated objects, changing diapers, or caring for infected people or animals such as calves, lambs, or goat kids.[1](https://www.cdc.gov/cryptosporidium/causes/index.html) Millions of oocysts can be shed in the feces of an infected person or animal, and shedding begins when symptoms start; a person remains infectious until two weeks after symptoms resolve.[1](https://www.cdc.gov/cryptosporidium/causes/index.html)
Higher-risk groups include international travelers, people using recreational water, caregivers of children ages 3 to 11 years, people in contact with pre-weaned calves, and people exposed to fecal matter through sexual contact.[1](https://www.cdc.gov/cryptosporidium/causes/index.html) In the United States during 2009 to 2017, treated recreational water accounted for nearly 35% of reported cryptosporidiosis outbreaks and almost 57% of outbreak cases, followed by contact with infected cattle (about 15%) and contact with infected persons in childcare settings (about 13%).[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) [Livestock](https://www.edgepedia.org/livestock) manure is a major environmental reservoir, with the global *Cryptosporidium* load in livestock manure estimated at approximately 3.2 × 1023 oocysts per year.[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full)
## Disease and treatment
Symptoms generally begin 2 to 10 days after infection, averaging 7 days, with watery diarrhea as the most common symptom; some infected people have no symptoms at all.[12](https://www.cdc.gov/cryptosporidium/about/index.html) In immunocompetent patients the illness is typically self-limiting, resolving within 2 to 3 weeks, and most people recover without treatment.[4](https://www.cdc.gov/dpdx/cryptosporidiosis/) • [8](https://www.cdc.gov/cryptosporidium/treatment/) In immunocompromised patients, including those with HIV/AIDS, organ transplants, or cancer, illness can be severe, prolonged, and life-threatening, with profuse watery diarrhea, malabsorption, and wasting; antiretroviral therapy can reduce or eliminate symptoms in people with HIV, but cryptosporidiosis is often not curable and symptoms can return if immune status worsens.[4](https://www.cdc.gov/dpdx/cryptosporidiosis/) • [8](https://www.cdc.gov/cryptosporidium/treatment/) • [13](https://www.chp.gov.hk/files/pdf/cdw_v22_1.pdf)
Nitazoxanide, available by prescription, is FDA-approved for people with healthy immune systems at least one year old; its effectiveness in people with weakened immune systems is unclear, and no effective drug exists for severe cryptosporidiosis in immunodeficient patients, young children, or neonatal livestock.[8](https://www.cdc.gov/cryptosporidium/treatment/) • [5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) [Diagnosis](https://www.edgepedia.org/diagnosis) relies on acid-fast staining, immunofluorescence microscopy, enzyme immunoassays, and increasingly PCR, which can identify the parasite at the species level.[4](https://www.cdc.gov/dpdx/cryptosporidiosis/)
## Public health burden
Cryptosporidiosis is a leading cause of diarrheal disease worldwide. In the Global Enteric Multicenter Study across seven sites in sub-Saharan Africa and [South Asia](https://www.edgepedia.org/south-asia), *Cryptosporidium* was the second leading cause of moderate-to-severe diarrhea in infants and the main cause of linear growth faltering.[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) The 2016 Global Burden of Disease study estimated 4.2 million disability-adjusted life years lost in children under 5 from acute illness, rising to 12.9 million when long-term effects such as growth retardation and cognitive defects were included.[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) In cattle, bovine cryptosporidiosis is endemic worldwide, with prevalence ranging from 11.7% to 78% across studies, highest in pre-weaned calves, and causes production losses through mortality, reduced weight gain, and veterinary costs.[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full)
**Waterborne significance.** The 1993 [Milwaukee](https://www.edgepedia.org/milwaukee) outbreak, in which 403,000 people developed diarrhea from drinking water that met federal safety standards, demonstrated the organism's public health importance and spurred research funding worldwide.[14](https://www.annualreviews.org/content/journals/10.1146/annurev.med.48.1.329) • [5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) Reviews of waterborne protozoan outbreaks have repeatedly found *Cryptosporidium* and *Giardia* to be the most prevalent waterborne protozoan pathogens globally.[15](https://www.sciencedirect.com/science/article/pii/S0048969721056394) A structured expert-judgment attribution study for the United States estimated waterborne transmission at 43% (uncertainty interval 17% to 73%) for *Cryptosporidium* spp., while finding that person-to-person contact and animal contact were also key pathways.[16](https://wwwnc.cdc.gov/eid/article/27/1/20-0316_article) Because oocysts tolerate standard chlorination, control depends on filtration, ozone, and ultraviolet treatment alongside hygiene measures.[5](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full) • [7](https://www.cdc.gov/mmwr/pdf/ss/ss6403.pdf)
## References
1. [How Crypto Spreads, CDC](https://www.cdc.gov/cryptosporidium/causes/index.html)
2. [NCBI Taxonomy Browser: Cryptosporidium](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=5806&mode=Info)
3. [Cryptosporidium Taxonomy: Recent Advances and Implications for Public Health, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC321466/)
4. [CDC DPDx: Cryptosporidiosis](https://www.cdc.gov/dpdx/cryptosporidiosis/)
5. [Khan SM, Witola WH (2023). Past, current, and potential treatments for cryptosporidiosis in humans and farm animals: A comprehensive review. Frontiers in Cellular and Infection Microbiology](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1115522/full)
6. [Cryptosporidiosis, StatPearls, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK448085/)
7. [Cryptosporidiosis Surveillance Summaries, MMWR](https://www.cdc.gov/mmwr/pdf/ss/ss6403.pdf)
8. [Treating Crypto, CDC](https://www.cdc.gov/cryptosporidium/treatment/)
9. [A Perspective on Cryptosporidium and Giardia, with an Emphasis on Bovines, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7103119/)
10. [Poop-encrusted chamber pots from the Roman Empire reveal oldest known human cases of Crypto parasite, Live Science](https://www.livescience.com/archaeology/romans/poop-encrusted-chamber-pots-from-the-roman-empire-reveal-oldest-known-human-cases-of-crypto-parasite)
11. [The cell biology of Cryptosporidium infection, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3130844/)
12. [About Crypto Infections, CDC](https://www.cdc.gov/cryptosporidium/about/index.html)
13. [Communicable Diseases Watch, Hong Kong Centre for Health Protection](https://www.chp.gov.hk/files/pdf/cdw_v22_1.pdf)
14. [Protozoal Agents: What Are the Dangers for the Public Water Supply? Annual Review of Medicine](https://www.annualreviews.org/content/journals/10.1146/annurev.med.48.1.329)
15. [Waterborne protozoan outbreaks: An update on the global, regional, and national prevalence from 2017 to 2020, Science of The Total Environment](https://www.sciencedirect.com/science/article/pii/S0048969721056394)
16. [Attribution of Illnesses Transmitted by Food and Water to Comprehensive Transmission Pathways Using Structured Expert Judgment, Emerging Infectious Diseases](https://wwwnc.cdc.gov/eid/article/27/1/20-0316_article)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Other microbial eukaryotes › Parasitic protists and protozoal disease › Apicomplexa*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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