Without succes I have searched the internet for studies about useful aspects of virus infections like a lower cancer risk for people who have had some childhood illnesses (chicken pox etc.).
Every hint is welcome.

I think the best known "useful" aspect of viral infection is that infection with cowpox virus provides immunity against smallpox virus... the origin of vaccination...

You might also try searching for web pages linking retroviruses with evolution...

Yes retroviruses have some intereting possible benefits.

Might want to look for stuff about correcting genetic problems, and also about viruses used to kill bacteria (bacteriophages).