<P> Trojan horses are generally spread by some form of social engineering, for example where a user is duped into executing an e-mail attachment disguised to be unsuspicious, (e.g., a routine form to be filled in), or by drive - by download . Although their payload can be anything, many modern forms act as a backdoor, contacting a controller which can then have unauthorized access to the affected computer . While Trojan horses and backdoors are not easily detectable by themselves, computers may appear to run slower due to heavy processor or network usage . </P> <P> Unlike computer viruses and worms, Trojan horses generally do not attempt to inject themselves into other files or otherwise propagate themselves . </P> <P> Once a malicious program is installed on a system, it is essential that it stays concealed, to avoid detection . Software packages known as rootkits allow this concealment, by modifying the host's operating system so that the malware is hidden from the user . Rootkits can prevent a malicious process from being visible in the system's list of processes, or keep its files from being read . </P> <P> Some malicious programs contain routines to defend against removal, not merely to hide themselves . An early example of this behavior is recorded in the Jargon File tale of a pair of programs infesting a Xerox CP - V time sharing system: </P>

A special type of software that is used to hide malware bots and worms