There is no possible virus when the infected system has perfect protection. The continuous existence of viruses is a reminder that perfection is rarely achieved in human endeavours.
If a virus or any piece of malevolent software succeeds in inserting its own public key in the trusted root store of your browser, then whoever controls the virus can make perfect impersonations of HTTPS Web sites, with fake certificates that your browser will accept as if they were the genuine thing. But, of course, why would the virus stop there ? The virus is hostile code running on your computer with at least as much privilege than your normal applications; the virus can simply open a door for external commands. Your machine is no longer, well, yours. When the attacker can put his own code on your machine, then grabbing your password when you connect to your bank site is much simpler than luring you to a fake bank site, so why would he bother ?