The Doomsday Book, by Connie Willis.

Winner of both the Hugo and Nebula awards. A time travel story set in both the future and the past. Historians in the future do field work in the past. Time travel is governed both by certain laws of nature and ethical codes that ensure that the past cannot be changed. One woman goes back in time to do her field work, but finds that the mission is jeopardized because she has fallen ill with a new strain of influenza that is about to ravage the future, which she has brought back with her to the past.

The story diverges from there, telling the tale of the scientist in the past and her colleagues in the future as they all deal with this new plague.

This got second place in last month's voting so I figured I'd nominate it again.