Re: "Try finding a virus that would both be capable of infecting a PDP-11"
minicomputer OS PDP-11
Any potential virus writers would first have to realise that PDP-11 isn't an OS, but a hardware architecture. I presume they're actually running some variant of RSX-11, though bits of the control system could also be running RT-11.
It would be wrong, though, to assume that these machines are standalone. There were Ethernet adapters for both Unibus and Q-Bus PDP-11s and there was manufacturer support for a wide variety of comms - DECnet, SNA, X.25 (the latter two developed in the UK) as well as (later) third-party support for TCP/IP. If there's still ongoing support, it's as likely they have the machines networked in some fashion as that they're doing updates by carrying RK05s from one machine to another.