2 3 1A p p l y i n g T h e s e M e t h o d s t o fi n d C a u s e sfind 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.^3
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.^4
Strictly speaking, of course, all these people who had contracted the disease
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 condi-
tion 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 spe-
cialized medical knowledge, a great many other conditions were also kept
off the candidate list.
One prime candidate on the list was presence at the Bellevue-Stratford
Hotel. The application of the NCT to this candidate was straightforward.
Everyone who had contracted the disease had spent time in or near that
hotel. Thus, presence at the Bellevue-Stratford could not be eliminated as a
necessary condition of Legionnaires’ disease.
The application of the SCT was more complicated, because not everyone
who stayed at the Bellevue-Stratford contracted the disease. Other factors
made a difference: “Older conventioneers had been affected at a higher rate
than younger ones, men at three times the rate for women.” Since some
young women (among others) who were present at the Bellevue-Stratford
did not get Legionnaires’ disease, presence at that hotel could be eliminated
as a sufficient condition of Legionnaires’ disease. Nonetheless, it is part of
medical background knowledge that susceptibility to disease often varies
with age and gender. Given these differences, some people who spent time
at the Bellevue-Stratford were at higher risk for contracting the disease than
others. The investigation so far suggested that, for some people, being at the
Bellevue-Stratford was connected with a sufficient condition for contract-
ing Legionnaires’ disease. Indeed, the conjunction of spending time at the97364_ch10_ptg01_215-238.indd 231 15/11/13 10:48 AM
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