[OT] Destroying all human life on Earth

[Gregor Richards]

You want to destroy all life on Earth. To do this, you're creating 
nano-robots. A single nano-robot cannot control a persons mind: Seven 
are required (three in each half of the brain and one in the brain 
stem). Nano-robots can harvest material from their host to build new 
nano-robots, but the host will die after approximately twenty 
nano-robots-worth of material has been harvested (they require 
particular rare particles that can only be harvested from the heart and 
lungs).

Robots can only be spread by direct physical contact from an infected 
host to an uninfected one, and the process of transferring one 
nano-robot destroys nano-robots (that is, the infected host loses three 
robots in the process but the new host only gains one).

Devise an algorithm for these nano-robots that will destroy all human 
life on Earth in the minimum amount of time. That is, minimize the time 
from deploying the first nano-robot to every human life being 
eliminated. You may assume a maximum of twelve degrees of separation 
between average industrialized people and that even the most remote 
tribe is connected by at least one human to the industrialized world.

Bonus: How would you change this algorithm if you wanted to destroy all 
animal life? All life?

  - Gregor Richards