Giardia
Giardia is a single cell parasite that lives in your gut and we get Giardia after eating contaminated food and drink. Giardia has a tough outer shell to survive the acid in your stomach and once in the gut it transforms and grows flagella (see trypanosomes) and is able to move and attach to our gut wall.
Image courtesy of http://phil.cdc.gov/PHIL_Images/8698/8698_lores.jpg