Here are two recent stories about viruses. They started out alike, and ended up very differently.

In October, a woman in Guinea died of Ebola, leaving behind two daughters, one of them two years old, the other five. A relative named Aminata Gueye Tamboura  took the orphaned children back to her home in northwest Mali–a 700-mile journey. Tamboura didn’t know it then, but the younger girl, named Fanta Conde, was infected with Ebola as well. For three days, they traveled on buses and in taxis as Fanta grew ill, developing a scorching fever and a perpetual nosebleed. Soon after arriving in Mali, she died.

Yet Tamboura never became infected with Ebola. Nor did Fanta’s sister or her uncle, who also made the trip. Nor did anyone