The phrase "harmless virus" refers to a type of virus that doesn't cause any damage or harm to a person or a computer. It might not create any problems, like making you sick or damaging your device. Full definition
In this technique, the otherwise harmless viruses are loaded with «vaccine» genes that make proteins identical to those in an infectious organism. (newscientist.com)
Whether a cat gets this devastating reaction to a normally harmless virus is more a function of the individual cat than about the virus. (marvistavet.com)
Some newer vaccine candidates use harmless viruses such as modified adenoviruses to deliver genes that encode immunizing viral proteins. (sciencedaily.com)