Viruses can be fairly complex, but imagine the very basic form of what they are: A string of DNA floating around and getting into cells, and causing them to produce more copies of itself. Obviously viruses are more than just rogue DNA, but that's their function - they corrupt other cells. They don't eat, photosynthesis, reproduce, etc. They're much closer to a chemical reaction, like how fire causes wood to create more fire, except orders of magnitude more complicated.