There is a fourth strategy, although it will need to be evaluated and deployed carefully. Since the 1990s, novel methods have allowed doctors to detect viruses in the pre-symptomatic phase of an infection, often with remarkable sensitivity and precision. One of these involves the polymerase chain reaction, or P.C.R., a chemical reaction that amplifies pieces of a virus’s genes floating in blood by more than a millionfold, which is what makes early, pre-symptomatic infections identifiable. The technique is not particularly cumbersome: As an oncologist working with blood cancers, I have been using variants of it to detect subclinical infections in patients for nearly a decade.

A study in The Lancet, published in 2000, illustrates the power of this approach. Twenty-four “asymptomatic” individuals exposed to Ebola were tested using P.C.R. Eleven of the exposed patients eventually developed the infection. Seven of these 11 tested positive for the P.C.R. assay, while none of the other 13 did. In 2004, virologists at the Centers for Disease Control and Prevention further refined this method to increase its sensitivity. The test now requires only a teaspoon of blood. The sample is transported on ice to a centralized lab. Results are back in a few hours.

Technologies like this allow us to imagine a new form of quarantine. Rather than relying on primitive instruments, indiscriminate profiling or questionnaires, we should consider running a pilot program to test asymptomatic travelers using sensitive P.C.R.-based techniques.