you don't need research to know your single person targetted virus or group of people is possible... you just need to make the virus a retrovirus that installs itself in stead of a specific DNA sequence making all the carryers of that DNA sequence the only targets...
(this is the same thing as gene therapy...)
modification of the sequence through mutation would surely affect other people, but the scale of this would depend on the lenght of the sequence (and thus the broadness of the initial virus).
The longer the sequence, the more specific, but also the least likely to mutate into a mass murderer virus. However, if your virus targets all the people with A blood type, it is more likely that a little mutation will make it target also B blood type...
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