I read that Mac has nasties that can't run in newer OS. 1) If OS is infected and new OS is patched against such infections or safe against them, will upgrading to new OS override those infections in old OS and... delete them? Seal them away? Keep the there, but harmless? Or will infections in old OS corrupt something during upgrade and infections stay active even when it looks like new OS was successfully installed? 2) And could infections in OS overall mess something up during OS upgrade and make it weak, cause security holes?