Are there any useful viruses, examples?

Viruses are not only harmful, but also useful. Moreover, many viruses are used in medicine for the production of enzymes, as well as for the treatment of infectious diseases, since many of them parasitize on pathogenic bacteria, leading to their death. Example: staphylococcal bacteriophages. Viruses are also of great importance for science. With the help of viruses, scientists transfer part of the genes from one organism to another, introducing new characteristics.