I think I have seen antivirus apps in the BB App world. There has only been 1 instance of malicious software ive seen which came from a legit app that gave out a update that spyed on users.
Also there are PDF BB exploits usually which affect BES users.
There have been reports of a few a while back but if your going though App world, you have nothing to worry about. But if your downloading from xyz.com, then it might pose a risk.