1. A virus writer sends out viruses, infecting ordinary users' PCs. 2. Infected PCs log into an IRC server or other communications medium, without their owners knowing, forming a network of infected systems known as a botnet. 3. A spammer purchases access to this botnet from virus writer or a dealer. 4. The spammer sends instructions to the botnet, instructing the infected PCs to send out spam. 5. The infected PCs send the spam messages to internet users' mail servers.