Silly? Lots of people think so, but one of the earliest researchers on computer viruses, Fred Cohen, recognized that a virus need not have malicious intent and that it might actually have good intent.
Consider that a large number of people don't have AV (anit-virus) protection despite being easily available and free. Wouldn't an AV program that spread itself from computer to computer be a win-win situation? Of course, that's assuming no bugs, that it doesn't over-use system resources, doesn't interfere with legitimate programs on the system, allows itself to be updated,... — and that's a lot to assume. Why not instead make the software freely available and let those that want its benefit to (easily) install it?
Such programs are available now. Some people still don't use them. That is how the last paragraph started - a circular argument.
It is a fun thought experiment, at the least. And computer experts have struggled over the idea for decades. Take a peek at some of the following: