Doomsday Book by Connie Willis

Kivrin, a history student at Oxford in 2048, travels back in time to a 14th-century English village, despite a host of misgivings on the part of her unofficial tutor. When the technician responsible for the procedure falls prey to a 21st-century epidemic, he accidentally sends Kivrin back not to 1320 but to 1348--right into the path of the Black Death. Unaware at first of the error, Kivrin becomes deeply involved in the life of the family that takes her in. But before long she learns the truth and comes face to face with the horrible, unending suffering of the plague that would wipe out half the population of Europe. Meanwhile, back in the future, modern science shows itself infinitely superior in its response to epidemics, but human nature evidences no similar evolution, and scapegoating is still alive and well in a campaign against "infected foreigners."

The book alternates between what is happening in the present and what is happening in the past, as those in the present work to unravel the mystery of what went wrong. Meanwhile, Kivrin struggles to overcome the anomalous situations she encounters that run contra to her expectations. Steeped in well-researched medieval life, it is the story of Kivrin's sojourn in the past that captures the imagination of the reader. This is a stunning book that is totally gripping