This is the fourteenth of 16 student posts, guest-authored by Caroline Rauschendorfer.
Cryptosporidiosis, known more commonly as crypto, is a gastrointestinal (GI) disease caused by parasites of the Cryptosporidium genus. If infected with crypto you may experience diarrhea, nausea, vomiting, fever, and abdominal cramps that can last up to two weeks. Definitely something you want to avoid, if possible, but at least it usually resolves on its own without medical intervention and is rarely fatal in otherwise healthy individuals. 
The most common disease causing organisms for crypto are C. hominis and C. parvum. C. hominis is a version of the parasite that sticks mainly to humans. Historically, large outbreaks of crypto tend to be caused by drinking water that has been contaminated with wastewater. In 1993 a large outbreak of crypto occurred in Milwalkee, Wisconsin, making 403,000 people ill. It was thought to be caused by a failure of drinking water purification. It is also possible to get crypto from a variety of other places as well: swimming in contaminated water, eating contaminated food, or from infected livestock. 
It is this last possibility for transmission that I am most concerned with. As I said earlier, C. hominis sticks primarily to people; but C. parvum, another type of Cryptosporidium, has the ability to infect most major domestic livestock.  As a second year veterinary student, this is a health concern for me. C. parvum is particularly common in cattle, and frequently causes infection in young calves. Crypto infected calves either come into a large animal clinic or are seen by a veterinarian on farm for being a “poor-doer” and having watery diarrhea. Veterinarians and veterinary students often come in close contact with these sick calves during treatment and can become infected themselves. As a matter of fact, contracting crypto as a veterinary student during your large animal rotation is practically a right of passage.
Many studies have been done looking at the prevalence of crypto in cattle, with a variety of results. One similarity between most studies, however, is that the prevalence of crypto is generally higher in calves than in adult cattle. Results from different studies ranged from 7.5 to 49% of U.S. dairy calves infected with crypto.  This means that working with calves puts you at an even greater risk of becoming infected with crypto.
No need to freak out yet, though. One thing that should be noted is: it is possible to come in contact with cryptosporidium and not become sick. Crypto is not difficult to kill, and will be removed from water by boiling it for at least 1 minute; however, routine chemical disinfection alone may not kill it. According to one study, crypto contaminates 65% to 95% of surface water and 45% to 65% of ground water in the U.S. This probably doesn’t sound like a good thing, but it actually might be. According to another study, 35% of the U.S. population has antibodies to crypto. Repeated exposure to small doses of crypto may help build immunity against it. 
The body’s immune system fights disease in two ways: adaptive and innate immunity. Innate immunity is the part of your immune system that you are born with and involves the cells that patrol and recognize any type of foreign cell or molecule that might be inside your body, such as bacteria, viruses and parasites. Adaptive immunity is something that the immune system builds over time as your body is invaded by various infectious agents (viruses, bacteria, parasites, etc.). The cells that are part of the adaptive system produce a memory when they are exposed to these infectious agents so that if they encounter them again, they can mark them to be killed by other cells. In other words, innate immunity works very generally and very quickly, whereas adaptive immunity adapts to the specific infectious agent, but it takes longer to kick in. This works for crypto in that if your body is exposed to the parasite for the first time as a small amount in your drinking water, it may not be enough to make you sick, but if your adaptive immune system finds it and makes a memory it can fight off the parasite much better if you are ever exposed to it again. This would also be true if you were infected with crypto and had all the gross symptoms that go with it, it would just be more unfortunate. 
Immunity is also the reason why crypto, although not serious for health individuals, can be very harmful for the immunocompromised such as those on chemo drugs for cancer treatment, or people with HIV/AIDS. The number of immune cells in an immunocompromised person can be so low that they do not have enough to fight off an infection. If someone with HIV/AIDS becomes infected with crypto, it is much more likely to be lethal. 
Treatment of crypto for someone who is immunocompromised would be to administer blood serum, which contains immune cells, from a healthy person. This has been shown to help in some cases. Unfortunately there is no treatment for otherwise healthy individuals who become infected with crypto. Your only option is to wait out the two miserable weeks until your body fights off the parasite. In some cases, IV fluids may be helpful for people who become dehydrated from severe diarrhea. 
Thankfully, there are many ways to help protect yourself from crypto. The CDC recommends, as always, washing your hands, especially before eating or preparing food, after gardening, after using the bathroom, after changing children’s diapers, after tending to someone who is ill with diarrhea, or after handling animals and animal waste. The also recommend keeping your child out of daycare if they have diarrhea. At pools, lakes, and any other recreational water source, they recommend: not swimming if you have diarrhea, especially children with diapers; showering before entering the water; washing children thoroughly after changing their diaper or after they use the toilet before allowing them in the water; and not changing diapers at the poolside. The CDC recognizes livestock handling as a potential risk for crypto infection and have special recommendations for these people: minimize contact with animal feces, particularly young animals; always wear disposable gloves when cleaning up animal waste and wash hands when finished; and wash hands after contact with animals or their living areas. As a veterinary student these are all things I will have to keep in mind while treating animals to help prevent two weeks of misery for myself. 
1. Centers for Disease Control. http://www.cdc.gov/parasites/crypto/gen_info/index.html. Updated 2010.
2. Dixon B, Parrington L, Cook A, et al. The potential for zoonotic transmission of giardia duodenalis and cryptosporidium spp. from beef and dairy cattle in ontario, canada. Vet Parasitol. 2011;175(1-2):20-26.
3. Leitch GJ, He Q. Cryptosporidiosis-an overview. J Biomed Res. 2012;25(1):1-16.
4. Preiser G, Preiser L, Madeo L. An outbreak of cryptosporidiosis among veterinary science students who work with calves. J Am Coll Health. 2003;51(5):213-215.
5. Tizard IR. Veterinary immunology. 8th ed. Philadelphia, PA: Saunders; 2008.