Epidemic!

Epidemic! is a strategy game in which the player stands in control of a world on the brink of extinction. A space-borne plague has hit the planet and the player must stem the epidemic as quickly as possible and with minimal loss of life. Your range of methods to deal with the epidemic range from inert methods like interferon, to quarantine, to the ultimate in immediate cessation, tactical nukes. The game has 4 difficulty levels, ranging from small controllable scenarios of approximately 20 minutes at level 4, to full one hour campaigns with severe difficulty at level 1.

Source: Wikipedia, "Epidemic!", available under the CC-BY-SA License.