Bots are often polymorphic, meaning that when they propagate to other machines, they create slightly different variations of themselves. This makes it hard for standard Antivirus software to be affective in identifying the bot. Bots can also disguise their presence on the infected machine by disabling antivirus defenses. By installing root-kits, the bot can give the appearance to the user that all is well even though the machine is compromised and its defenses are down.