Suppose you are working with James Lind in 1747, attempting to isolate the cause of scurvy. What: After a long period at sea, a number of crew members are suffering from scurvy, characterized by weakness, fatigue, listlessness, bleeding gums, loose teeth, and subcutaneous bleeding. Where: You note that scurvy most often occurs aboard ships, never in a fertile countryside. Although the hold is stuffy, sailors get considerable fresh air working topside. Food and human waste are emptied over the side daily, not remaining to decay. Water and food are stored and are the same for all, except that the officers eat better. Bathing is available frequently, but in seawater, not freshwater. When: Scurvy never occurs at the beginning of a voyage, but always after a lengthy time without landfall. Once begun, scurvy only worsens aboard ship and improves only on return to shore. Who: Anyone aboard ship is susceptible, but the incidence rate for officers is much lower. Crew members are in close contact over extended periods. Some contract it, but others do not. Therefore, you conclude that it is not contagious. (You have no knowledge of other limits to contagion such as immunization.) How: How do you further your investigation?