Better go on a leave - for, oh, say at least 21 days.
21 days? Perhaps that will give one an idea of the risk.
But, it has been 21 days since Duncan first arrived at the hospital. And this second nurse just shows up today. If she infected someone they may not show until 21 days. But the person that someone may have infected may be another 21 days (so now 42 days). Etc.
And worse - the nurses and doctors and other patients in the hospital with Duncan knew that they were at risk, and were monitoring themselves. While this nurse was an idiot for travelling, at least she went to the hospital before she was violently ill.
How many others, not knowing they were exposed, will figure the fever is just a fever? And the vomiting is just the flu? “Oh honey - now I’m vomitting blood and can’t even stand I’m so weak. Call a cab!”