Time Travelers (1976)

When a novel virus devastates the world in 1976, Dr. Earnshaw notices that it resembles a virus that a Dr. Henderson found a cure for in 1871 Chicago. However, the doctor perished and his notes were destroyed in the Great Chicago Fire. Earnshaw is approached by a government agent, Jeffrey Adams, who informs him of a Top Secret time traveling technology and asks for his assistance in finding Dr. Henderson and his cure. But their adventure becomes a race against time when a glitch sends them back to the day before the fire instead of the planned four days.