This is not so much a writing exercise as a puzzle. It is the type of puzzle that epidemiologists do to discover the source of an infection, or patient 0. Pathogens can be transmitted through the air, on inanimate objects, in water or food, by insects or by direct contact between people. In our epidemiology problem, over three or four days, 11 people get sick enough to go to a local hospital with severe diarrhea and vomiting that lasts four days or so in each patient. The disease is serious enough and unique enough that an intern at the hospital becomes curious about the small disease cluster, and she tests the patients for norovirus and bacterial food contaminants, including Salmonella, E. coli o157:H7 and Campylobacter. All the patients turn out to all have the same strain of norovirus. The intern asked a few questions to see what the patients had in common, and it turned out that they all knew each other and over the summer had been sharing produce from their gardens. The intern asked what, specifically, they had shared for fresh, uncooked produce over the last week, who they had shared with, and when. Her hypothesis was that one person had norovirus, and had transmitted the virus to others on the food. She made a list, numbered the patients, starting with the patient that had first shared, and who they had shared with. It turned out a total of 16 people had shared produce, so she contacted the additional people who had not gotten sick, and asked them who they had shared produce with and when. In the end, she came up with the list below. So, patient 1 first shared vegetables with patient 12, then with patient 14. Patient 2 first shared vegetables with patient 5, then with patient 15, and so on. And patient 1 never got ill, while patient 2 did. Any time that two people come in contact with each other, the virus can move either way. For example it would be possible for patient 2 to have infected patient 5, or patient 5 to infect patient 2. After studying the list, she said, “I know who started this!” She asked that patient where they had been recently and it turned out they’d been on a cruise ship that had had a severe outbreak of norovirus! Based on her data, which patient was the one who went on the cruise and started the epidemic?”