After their research was finally completed, with the bacterium identified, described, and named, it was then said that Legionella pneumophila was the cause of Legionnaires’ disease. What was meant by this? To simplify a bit, suppose L. pneumophila (as it is abbreviated) entered the bodies of all those who contracted the disease: Whenever the disease was present, L. pneumophila was present. Thus, L. pneumophila passes the NCT for the disease. We will further suppose, as is common in bacterial infections, that some people’s immune systems were successful in combating L. pneumophila, and they never actually developed the disease. Thus, the presence of L. pneumophila would not pass the SCT for the disease. This suggests that we sometimes call something a cause of an effect if it passes the NCT for that effect, even if it does not pass the SCT for that effect.
But even if we sometimes consider necessary conditions to be causes, we certainly do not consider all necessary conditions to be causes. We have already noted that to get Legionnaires’ disease, one has to be alive, yet no one thinks that being alive is the cause of Legionnaires’ disease. To cite another example, this time one that is not silly, it might be that another necessary condition for de- veloping Legionnaires’ disease is that the person be in a run-down condition— healthy people might always be able to resist L. pneumophila. Do we then want to say that being in a run-down condition is the cause of Legionnaires’ disease? As we have described the situation, almost certainly not, but we might want to say that it is an important causal factor or causally relevant factor.