Character of the Illness

The initial step in the investigation of any epidemic is to determine the character of the illness, who has become ill and just where and when. The next step is to find out what was unique about the people who became ill: where they were and what they did that was different from other people who stayed well. Knowing such things may indicate how the disease agent was spread and thereby suggest the identity of the agent and where it came from.

Part of this procedure involves a straightforward application of the NCT: Was there any interesting feature that was always present in the history of people who came down with the illness? Progress was made almost at once on this front:

We quickly learned that the illness was not confined to Legionnaires. An additional 72 cases were discovered among people who had not been directly associated with the convention. They had one thing in common with the sick conventioneers: for one reason or another they had been in or near the Bellevue-Stratford Hotel.

Strictly speaking, of course, all these people who had contracted the dis- ease had more than one thing in common. They were, for example, all alive at the time they were in Philadelphia, and being alive is, in fact, a necessary condition for getting Legionnaires’ disease. But the researchers were not interested in this necessary condition because it is a normal background condition for the contraction of any disease. Furthermore, it did not provide a condition that distinguished those who contracted the disease from those who did not. The overwhelming majority of people who were alive at the time did not contract Legionnaires’ disease. Thus, the researchers were not interested in this necessary condition because it would fail so badly when tested by the SCT as a sufficient condition. On the basis of common knowledge and specialized medical knowledge, a great many other conditions were also kept off the candidate list.