"Ebolanoia" and the Fallacy of Composition

In "The Psychology of Irrational Fear," Olga Khazan ponders why Americans are afraid of Ebola when the flu presents a more immediate danger. She offers several reasons for "Ebolanoia" (e.g. that we're afraid of the unfamiliar), and like most pundits, she takes it for granted that fear of Ebola is irrational on the grounds that the chance of an American contracting Ebola is very near zero. Let's formalize the idea behind this thinking:

It is irrational for S to fear P when the probability that P will harm S is nearly zero.

Although this principle is true when S is obsessively concerned about being harmed by P, it is not simpliciter irrational for S to fear P even when P presents no immediate danger to S. For example, I'm not concerned about being killed by a tornado. However, I am concerned that someone somewhere will be killed by a tornado. Indeed, an average of 60 people in the United States are killed by tornadoes. That's why we have an elaborate warning system in place. Likewise, I'm not afraid of being killed by terrorists, but I am afraid of terrorists attacking someone somewhere in the United States.

What's gone wrong with the above principle is that it commits the fallacy of composition. This fallacy occurs when the properties of individual parts are assumed to be properties of the whole. For example, if every member of a club is 20 years old, it doesn't follow that the club itself is 20 years old. Likewise, the chance that any single American will be killed by a tornado is very low, but it doesn't follow that the chance of the United States experiencing a deadly tornado is very low. Indeed, it is almost certain that the United States will experience a deadly tornado in the future.

To be sure, Ebola does not present the same threat to the United States that tornadoes do--and probably never will--but given our rather laissez-faire approach to quarantine and the CDC's ineffective response to Ebola in Dallas, it is rational to be concerned about Ebola spreading. Initially, the conventional wisdom held that Ebola would not develop in the United States. Then two nurses contracted Ebola from treating an Ebola patient, prompting CDC director Thomas Frieden to admit that "Stopping Ebola is difficult." It's no wonder, then, that many Americans are concerned--not hysterical--about the presence of Ebola in the United States.