Trojan-Trojan viruses slip into an individual’s system and run without the user’s knowledge. They can have many malicious functions. For example, some trojans are dialers that use a computer's modem to dial long-distance, generating huge phone bills for the computer owner. Unlike viruses and worms, trojan infections do not make copies of themselves.

They are often used to exploit a user's computer to download other malicious adware and spyware programs, through  a backdoor or a downloader mechanism.