I was curious also, so I googled “beneficial virus examples” and this was the top result. https://www.sciencemag.org/news/2013/05/friendly-viruses-protect-us-a

I was curious also, so I googled “beneficial virus examples” and this was the top result. https://www.sciencemag.org/news/2013/05/friendly-viruses-protect-us-against-bacteria https://www.sciencemag.org/news/2013/05/friendly-viruses-pro... > Bacteria can be friends and foes—causing infection and disease, but also helping us slim down and even combating acne. Now, a new study reveals that viruses have a dual nature as well. For the first time, researchers have shown that they can help our bodies fight off invading microbes.