Active Bot-Infected Computers per Day

Between January 1 and June 30, 2007, an average of 52,771 active bot-infected computers were observed per day, a 17% decrease from the previous reporting period. The decrease in bots observed over the past six months is likely due to a number of reasons, the primary one being a change in bot attack methods. The exploitation of network-based vulnerabilities to spread bots is being slowly abandoned for methods that are more likely to succeed, such as bots that send a mass mailing of themselves.

Source: Symantec Internet Security Threat Report, Vol. XII