From everthing I have read, once the Ebola symptoms show, it is downhill VERY quickly (hours). She probably had a fever from something else, or a “sick” feeling due to anxiety from being near Duncan. Regardless, she should have known better to expose others in case those early symptoms were Ebola. (But - I really don’t think they were.)
Duncan went to the hospital, was tuned away, and came back days later. He theoretically could have infected a ton of people if he wanted to.