You are thinking "trojans," not viruses. Viruses spread without any user interaction by exploiting vulnerabilities in clean, but accessible machines. Ransomewa

You are thinking "trojans," not viruses. Viruses spread without any user interaction by exploiting vulnerabilities in clean, but accessible machines. Ransomeware used to be purely trojans, but newer iterations now come with a viral component based (at least partially) on that leaked NSA toolkit.