Did you know that some viruses can be upgraded by add-on software updates? Some of such updates, like hepatitis D, are mere patches amending the gameplay of the base virus; in this case, hepatitis B. These are called satellite viruses. But there's also viral DLC that doesn't even have its own box, and persistently steals the genes to make the capsid for itself from another virus. These are called viroids, and perhaps unfortunately, you would be unlikely to have heard of any unless you're into plant medicine (or arcane genomics), becuse there aren't any viroid diseases known (yet) that infect any creature that is not a plant. But plants, including many plants that human farmers care about because they make delicious edibles when healthy, can suffer from a large variety of viroid diseases, so #MoreResearchIsNeeded.