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The diplomonad protist, Giardia lamblia, colonizes and replicates in the small intestine of mammals. In humans, Giardia infection (known as giardiasis) can be symptomatic or asymptomatic with the former being associated with fatty stool, abdominal pain, malnutrition, and weight loss. Both cell-mediated and humoral immune responses against Giardia infection are possible, and adaptive responses have been reported to be important for controlling the infection. Non-immune components such as secretory immunoglobulin, intestinal lipids, and bile acids also play significant roles in determining the severity of the infection by Giardia.

Giardia exists in two morphologic forms--trophozoites and cysts--and maintains a simple life cycle. Exposure of cysts to gastric acid during passage through the human stomach triggers excystation (morphological changes from cyst to trophozoites), while factors in the small intestine, where trophozoites colonize, induce encystation or cyst formation. The disease is transmitted by water-resistant cysts, which are excreted in the feces of humans and pass on to a new host via contaminated food and water. The production of viable giardial cysts is critical for successful transmission and spreading of the disease.