Legionnaires’ Disease

Appealing to background knowledge once more, it seemed that a bacterium would be the most likely source of an airborne disease with the symptoms of Legionnaires’ disease. But researchers had already made a rou- tine check for bacteria that cause pneumonia-like diseases, and they had found none. For this reason, attention was directed to the possibility that some previously unknown organism had been responsible but had some- how escaped detection.

It turned out that an undetected and previously unknown bacterium had caused the illness, but it took more than four months to find this out. The difficulties encountered in this effort show another important fact about the reliance on a background assumption: Sometimes it turns out to be false. To simplify, the standard way to test for the presence of bacteria is to try to grow them in culture dishes—flat dishes containing nutrients that bacteria can live on. If, after a reasonable number of tries, a colony of a particular kind of bacterium does not appear, then it is concluded that the bacterium is not present. As it turned out, the bacterium that caused Legionnaires’ disease would not grow in the cultures commonly used to detect the presence of bacteria. Thus, an important background assumption turned out to be false.

After a great deal of work, a suspicious bacterium was detected using a live-tissue culture rather than the standard synthetic culture. The task, then, was to show that this particular bacterium in fact caused the disease. Again to simplify, when people are infected by a particular organism, they often develop antibodies that are specifically aimed at this organism. In the case of Legionnaires’ disease, these antibodies were easier to detect than the bacterium itself. They also remained in the patients’ bodies after the infection had run its course. We thus have another chance to apply the NCT: If Legionnaires’ disease was caused by this particular bacterium, then whenever the disease was present, this antibody should be present as well. The suspicious bacterium passed this test with flying colors and was named, appropriately enough, Legionella pneumophila. Because the investi- gators had worked so hard to test such a wide variety of candidates, they assumed that the disease must have some cause among the candidates that they checked. So, since only one candidate remained, they felt justified in reaching a positive conclusion that the bacterium was a necessary condi- tion of Legionnaires’ disease.