Is there a practical difference between intentionally targeting cancer patients and intentionally targeting proxy signals for cancer patients? This is the whol

Is there a practical difference between intentionally targeting cancer patients and intentionally targeting proxy signals for cancer patients? This is the whole part of internet marketing targeting right? You don't know for certain if someone is a cancer patient without their medical records, so you record a load of signals and infer that someone is likely to require cancer medication in the absence of absolute information.