Flagellated protozoan
Most common cause of parasitic diarrhea worldwide
Transmitted by water contaminated with feces (human, beaver, muskrat, dogs, raccoons, etc)
Common among campers and is also known as “backpacker’s diarrhea”
Common in travelers to former Soviet Union, Caribbean, Latin America, India, Africa
Infection rate is twice as high during summer months
Also may be transmitted by contaminated food or close physical contact (sexual activity, daycare centers, etc)
Patients with decreased gastric acidity, immunoglobulin deficiency, or immunocompromise are more susceptible
Other names: “beaver fever”, “the Trotskys” (common in travelers to Leningrad)   
Trophozoites infect duodenum, jejunum, and ileum where they form cysts
Cysts are passed in feces; viable for long periods of time
A single diarrheal stool may contain hundreds of millions of cysts or parasites
Giardia lamblia
Viral 
Helicobacter pylori
Bacterial
Balantidium coli
Campylobacter
Clostridium difficile
Shigella