RAW doesn't really define whether genetics exist, or virus or bacteria, and that might really depend on yours setting. I think if it requires 20th or 21st century knowledge to distinguish it further, we should just treat it as a "disease".
I think one could make a case for something like shizophrenia or PTSD to not be treatable with Remove Disease, because medieval and fantasy people probably would think of this is some problem with the mind or soul and not of physical causes, even though through our knowledge of brain chemistry, hormones and what not we might now have drugs to treat something like that, in addition to psychological therapies.
But maybe it should be wholly setting-dependent. I could see that in the Witcher Universe, where people know about mutations and genetics and I think even atoms, such distinctions might be possibly and made. But mabe that's not a setting I'd try to emulate with D&D.