SIX APART HAD FALLEN PREY to a botnet a network made up of independent programs, or bots, acting in concert. Over the years, corporate IT managers have learned to firewall their networks to block unauthorized intrusion and patch their system software to keep out viruses, worms, and Trojan horses. Likewise, PC owners have installed tens of millions of personal firewalls and antivirus programs. But bots are infiltrating even protected computers, and they have quickly become a bigger threat than virulent malware like the famously destructive Melissa, I Love You, and Slammer.