A 30-year-old hiker presents with a 2-week history of foul-smelling, greasy diarrhea, abdominal bloating, and fatigue. He reports drinking untreated water from a mountain stream during a recent hiking trip. Physical examination is unremarkable. The clinician suspects giardiasis and investigates the mode of transmission. Which of the following is the most common mode of transmission for Giardia duodenalis?