Apparently, you can write a virus that can go through the power cord on a laptop- not a power line networking device, but the actual power cord- to infect other systems. This was actually how it was said to have accessed the NCIS network.
To give them more credit than they deserve, the operating state of an electrical device can generate some current fluctuation in its power cord, and in severe cases, even in other devices on the same circuit. If the circuit is anywhere near competently designed and constructed, this will be a very small fluctuation, but to be fair it would exist.
But to exploit it to the effects seen in tonights episode, the only way this is even in the same multiverse as plausible would require specific targeting of the network being attacked and the vector being used. You’d need to know exactly how your vector performs under varying operating conditions to reliably generate a signal, you’d need to know exactly how the circuit it’s attached to is designed and implemented, you’d need to know how other devices attached to it will affect things- someone plugging in a coffee machine you didn’t expect could throw the whole thing off by changing how the circuit reacts to what you do to it. An electrician using an extra inch of wire could throw it all off. Microamps of current draw could throw it off.
I really wouldn’t be surprised if this would require such precise control of the currents you induce in the power cord that quantum mechanics makes it more random luck than the virus actually doing anything, unless the virus writer is waiting to hear back on his Nobel Prize in EVERYTHING FOREVER for coming up with a reliable means to predict quantum effects with absolute precision.
This isn’t even getting to the issues of how to jump from one circuit to another, or how the outbreak was restricted to just the NCIS network when it somehow could propogate over power lines without actual power line networking fast enough to take out their entire network in a second or two.