Giardia is a common cause of diarrhea in dogs. It is a protozoan parasite that dogs can become infected with by ingesting cysts from contaminated water, food, or contact with the feces of infected dogs. The most common symptom is diarrhea, which occurs about two weeks after infection. Diagnosis involves identifying the cysts in a fecal sample using techniques like zinc sulfate centrifugation. Treatment involves oral medications like metronidazole, fenbendazole, or albendazole for 2 to 10 days. Preventing spread involves cleaning and disinfecting areas the infected dog inhabits, isolating infected dogs, and testing and treating dogs in kennels with diarrhea.