Quantum Computer Security???

I know quantum computers aren't commercially available yet, but when I first heard about them someone had said it would be impossible to create viruses for it. They would simply be too secure. But later heard that it would be possible. Does anyone know the ins and outs of quantum computer security?
Havn't heard about impossible to get viruses
But quantum encryption would (theoretically) be impossible to crack, making it quite handy for anything needing secure communication
I just remember in the days before Win95 when viruses were far less of a thing and most people didn't need internet to get most of their work done. It was a good time. Lol.