Erm, not sure what you mean by 'rubella' - other than meaning German measles, this word is usually an appendage to a plant name, as in Skimmia rubella (meaning reddish, or rubyish).

As for explaining why your nameless shrub has died after 15 years, without knowing what it actually is and any symptoms leading up its death, difficult to say.

Could you supply a photo?

Do you mean Ribes, the flowering currant?

Are you referring to an evergreen shrub? In any case fifteen years is a good run and you will never know the real cause of its end.