Reasons
1) Most DNA is same with sparse deltas, so pathogen would need to match large amount of DNA and ignore lots. Challenges in biometrics and general mirror this.
2) Things tend to mutate, so even if this was possible with a single pathogen, it may mutate either targeting other genetics, or changing behavior.
3) Targeting someone specifically, and getting the "germs" into their system is tricky. I haven't thought about this as much but high mutation rate means that it would need to be introduce physically and socially close to target.
Maybe targeting specific racial groups or some other evil thing would be easier. There are many existing disease which affect different groups (not just racial) in differently with statistical significance.
That being said, I hadn't thought much about this before and it's certainly a "fun" scary thought.