Trojan-

A Trojan Horse computer virus gets it name from the infamous legend of the same name. Just like the Trojans were fooled into letting opposing forces in their gates, a Trojan Horse virus is sneaky. This type of virus disguises itself as a helpful program.

Normally, a Trojan virus is distributed in this way:
-A user downloads a program from the Internet because they think it may be useful.
-Once the program is opened (run) on the user’s computer, the virus is released. Commonly the virus will erase the hard drive or destroy parts of the computer.
-Sometimes a Trojan will hide on the computer undetected, and slowly start transforming files and documents. This way the user does not know how or where they downloaded the virus.