Trojan horses are a huge security threat. A Trojan is a program that enters
your computer undetected, giving the attacker who planted the Trojan unrestricted
access to the data stored on your computer. Trojans can transmit credit card
information and other confidential data in the background. Trojans are often
not caught by virus scanning engines, because these are focused on viruses,
not Trojans. Catching such threats would require the use of a Trojan scanner
(a.k.a Trojan cleaner, Trojan remover, anti-Trojan).