"it doesn't just burrow along the cables, once on one PC in the Network. "
Sorry, that's precisely what some viruses can do (the CryptoLocker ransomeware being the latest example), assuming that we're talking about networked connections.
The OP asked about scenarios involving multiple PCs and one monitor or printer. Those are fine since, assuming we're not talking about a networked configuration, the computers are considered standalone. However, as soon as computers become networked they are potentially vulnerable to being infected by another PC on the network. It all depends on whether the particular virus is programmed to automatically spread via network stacks/shares and what 'network lockdowns' are in place. For example, if you're unlucky enough to be infected by a CryptoLocker variant, not only will it encrypt your files but, by default, it will automatically encrypt files on any other network PC drives which are shared and which you have access to. Also it will encrypt server shares, as well...very not nice!
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