Now mind you, this was 13 or so yrs ago, before viruses were well known or rampant. Anyway, we had the pleasure of without hesitation saying "no," because we're working on a mainframe system w/such and such an OS, and there are no viruses that can infect such a system. He asked us if we were sure. We looked him in the eye and said "yes. positive." and we left. That was pretty much our purpose for flying to his base - that one question which he wanted to ask us face to face.
Today systems are flimsy and pc-server based -- cheap alternatives to the real thing, chosen by people trying to save money and use lower salaried, less-experienced network people (vs. the higher paid mainframe/midrange experts). When it comes to such an important system, our power grid, there should be no such scrimping. Just like w/the Navy Admiral who took the safety of his men and resources seriously.