Dooms Day Book by Connie Willis

If you love historical fiction, this gripping tale is for you. Young historian Kivrin is dying to see what life was really like in the Middle Ages, even though her mentor, Dunworthy, thinks neither she nor the time-travel program are quite ready for such a dangerous journey. But he's overruled and back she goes--only to be trapped in this turbulent time period when the team assigned to retrieve her is struck down by a mysterious virus. Willis has clearly done her research on how hard life really was in the 1300s and when the bubonic plague strikes, you wonder how anyone will manage to survive, including our intrepid Kivrin. Stayed up till 1 am to finish it.

I can also highly recommend To Say Nothing Of the Dog by the same author.