You are mistaken.
The Ebola virus can be contagious when the infected person exhibits no signs, and beyond the 21-28 days used.
I'm not sure that's true.I went to East Africa last fall and before leaving I went to the travel clinic at Massachusetts General Hospital for malaria medication and advice.I told the doctor,an Infectious Diseases specialist,that I intended to stay away from family and friends for 21 day after my return (to protect them from possible harm) and asked him if I should say away longer.He said no,if I was feeling well 21 days after leaving the area that meant that everything was OK.
MGH is the most important teaching hospital of Harvard Medical School