Patient Zero (The Outer Limits)

Plot

An epidemic has broken out across Earth, and most of humanity has been killed by a virus. The virus began with patient zero, a woman who comes into contact with the three DNA strands necessary for this virus to come into existence. A soldier, Colonel Beckett, is sent back in time to kill her and prevent the virus from forming.

Beckett becomes attached to the woman and decides not to kill her, but instead to simply keep her from making the contacts necessary to form the virus. In the revised timeline, Beckett himself becomes patient zero - in his attempt to protect the woman, he himself contracts all three strains and becomes sick. A fellow soldier arrives from the future, and Beckett is given the chance to sacrifice himself when his friend administers a fatal dose of poison. Since Beckett dies before the disease reaches its contagious stage (and the woman never develops the disease at all), the plague is stopped before it starts - in the new history, there never was an epidemic, all of the disease's former victims (including Beckett's family) are now alive, and humanity survives without ever contracting the virus.