Summary: A single, immortal alien life form that kills by touch seeks to eradicate all life on Earth as efficiently as possible. After killing a victim, he is able to sense the next human nearest to him, and begins to walk toward him at 3 mph until making contact, and thus killing him. The alien's speed increases at a rate of 1 mph per hour that he does not kill. How long can mankind hope to survive before extinction?
Additional details: (1) The alien is invincible, immortal, doesn't require sleep, and can travel effortlessly through any terrain (including bodies of water) or blockade. (2) Take general population distribution into effect if possible. (3) Consider that humans, once reasonably aware of the threat, can assume reasonable strategic patterns of behavior to avoid death (as individuals or groups). They cannot alter the speed of the alien, but might position themselves in such a way as to survive for as long as possible.