A protozoa (CDC). There are a variety associated with different animals, all of which can infect humans (PubMed): C. hominis (human), C. parvum (human and cattle), C. meleagridis (birds), C. canis (dogs), C. felis (cats) C. cuniculus (rabbits), and C. suis (pigs).
Drinking contaminated water. Associated with cattle farmers. And swimming pools and water parks, where it causes outbreaks, as it is fairly resistant to chlorine (PubMed) (MMWR). I can survive 10 days in chlorinated water. Just what do you think kids are really doing in the pool?
Petting zoos can also be a source. What end of the animal are they petting? (PubMed)?
In Uganda it is a respiratory pathogen, perhaps spread by coughing (PubMed).
Diarrhea, self limited in normal people but lasts a week or more of watery diarrhea, refractory disease in AIDS.
The only approved drug is nitazoxanide 500 mg BID x 3 days in normal adults; efficacy and dosing in the immune compromised unknown.