4 : a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs and that usually performs a malicious action (as destroying data)
http://www.webster.com/I have lost data to virus scanners, they are often hidden in their function in windows, it auto-updates itself and can self-propagate over a network...
I see them as the ultimate virus... turn it off and watch your desktop turn into a supercomputer!
(P.S. not to mention the classic conspiracy theory of who is better motivated to develop (support) viruses than virus scanning software company employees... one of the few industries that can artificially create a real need/demand for its products... defining your own job security... too tempting to be resisted if you ask me...)