Routes of infection in spyware

Spyware does not directly increase in the manner of a computer virus or worm: generally, an infected system does not attempt to transmit the infection to other computers. Instead, spyware gets on a system through deception of the user or through development of software vulnerabilities.

Most spyware is installed not including users' knowledge. Since they tend not to install software if they know that it will interrupt their working environment and compromise their privacy, spyware deceives users, either by piggybacking on a piece of popular software such as Kazaa, or tricking them into installing it (the Trojan horse method). Some "rogue" anti-spyware programs deception as security software, while being spyware themselves.