David Baltimore -- Introduction to Viruses

Wait...if viruses can only replicate by hijacking cells, a precarious form of reproduction, wouldn't that suggest that for every virus we know about there must be....I dunno....millions...that are never successful (or yet to be successful)? Where do they come from? Are they simply mutations that can transfer between species? Why would nature produce something that is so unsuccessful and yet periodically so devastating? Also, wouldn't a vaccine work the same way (meaning rarely) and at similar rates to a virus? Feels like Humanity should be studying these a lot more than we apparently do.