The SQL Slammer worm, also known as Sapphire, was the fastest thing on the net. According to a fascinating report here, it infected more than 90% of vulnerable hosts within 10 minutes. "One implication of this advance [in speed] is that smaller susceptible populations are now vulnerable to attack. Formerly, small populations (<20,000 machines or less on the Internet) were not viewed as particularly vulnerable to worms, as the probability of finding a susceptible machine in any given scan is quite low. However, a worm which can infect a population of 75,000 hosts in 10 minutes can similarly infect a population of 20,000 hosts in under an hour. Thus, exploits for less popular software present a viable breeding ground for new worms."