Virus's require critical density in order to spread. The same way the flu runs rampant through crowded urban areas today, viruses only spread quickly when ther

Virus's require critical density in order to spread. The same way the flu runs rampant through crowded urban areas today, viruses only spread quickly when there's enough computers for them to infect. The lack of standard issue malware is probably due to better (although not perfect) security, and lack of economic incentives due to lower market share.