While that "full of virus" comment was made in jest, there's some truth in that, for some programs installation may need to be invasive, and will be blocked by anti-virus program. In effect, the installation is acting like a virus even though it is far from being intended as one. Some AV is stricter than the other, instead of naming names they just instruct you to turn it off.
Just like a gun, it can be used as a weapon or security device by a soldier or police, or being used by a murderer and a psycho. How would you know which is it going to be in advance?