DALLAS—The Ebola crisis in the United States escalated Wednesday, as a second nurse here was diagnosed with the disease, concerns rose over whether more health care workers at a Dallas hospital might be infected, and President Obama faced increasing questions about whether the U.S. government is doing enough to ensure Americans' safety.

People with Ebola are contagious only when symptomatic and get more contagious the sicker they become, so it's unlikely that Vinson was particularly contagious on Monday night. But all her fellow passengers on that flight will be notified that they were potentially exposed to Ebola. It is not clear whether they will be asked to monitor their temperatures twice daily for 21 days—the duration of the virus's incubation period—and