However, you still can get a virus from an exploit one without a patch deployed or a zero day. This require no action of your own. Although keeping your system

However, you still can get a virus from an exploit one without a patch deployed or a zero day. This require no action of your own. Although keeping your system up to date lessens the risk. It's not zero though. Moreover, if your running anything that is not very niche or custom system people are probably targeting it. I am not sure I would call BSD niche enough. Also, even if you are using a niche system that does protect you against a targeted attack. If you were a government a target attack is much more likely. Honestly, unless you have 100% bug free software and hardware, and its designed with security in mind. The possibility of malware taking advantage of bugs exists. Moreover, that excludes social engineering and getting someone to do something stupid. Even if the system normally is extremely secure. If we are getting super serious considering all the firmware that exists in a computer. If a system ever ran code you did not audit you can never be sure that system is secure. Even if you have an AV. You would have to re-flash any firmware that could be written to by the CPU, and reinstall the OS as soon you ran code that you did not audit. I doubt there really is any system except small embedded systems that you can audit all the code that closely. Modern systems just have too many layers.