The Warhol Worm: A theoretical piece of malware that could infect all the vulnerable servers on the Internet in 15 minutes.
But with 1 million vulnerable machines, any given scan and probe has a .025% chance of being a vulnerable machine. Thus, with the 1.2 million scans per second the initial worms send out, 300 will reveal new targets. By the second minute after release, the worm will have infected a total of 30,000 machines. After the third minute, there will be over 70,000 infected machines. It becomes quite obvious that complete infection will be achieved within the 15 minute timeframe.
Normally, once a worm which uses random probes infects about 1/2 of the available hosts, the rate of new infections slows down considerably. A fully coordinated worm, where the tasks of scanning the internet are perfectly divided, will only slow down once every target is infected. The pseudo random/random combination is a compromise, allowing the worm to do a comprehensive scan of the internet without relying on transmitting information between worms, just the ability to check to see if a potential target is already infected.