Anaerobic protozoan parasites
Found in the human gastrointestinal tract 
Most common eukaryotic parasites found in human stool
Controversial if they are a commensal organism or a pathogen
Have been found worldwide
Reside in the cecum and colon of both children in adults
Have also been found in various animals (including pigs, monkeys, rodents, and poultry)
Prevalence is geographically variable, but generally higher in developing countries
Mode of transmission is not fully understood, but though to be fecal-oral 
Vary in size between 5 and 40 micrometers
Lack a cell wall
Have mitochondria, golgi apparatus, smooth and rough endoplasmic reticula
Typically reproduce by binary fission
Grow in anaerobic culture conditions
Blastocystis species
Viral 
Cryptosporidium
Bacterial
Cyclospora
Campylobacter
Eikenella
Shigella