I explained the same thing to my supervisor at work. Although, I threw in the caveat that I'd look it up during break just because I kept hearing about new cases in the United States despite having been previously certain ebola wouldn't be an issue here. When I did look it up, I explained that the new cases were all nurses who had been around someone at the time when they'd been vomiting heavily, and that prior to being admitted, said patient had been around dozens of people who did not catch the virus. I think that calmed her down.