In the USA, health plans aren't allowed to deny coverage to patients based on genetics or pre-existing conditions. They aren't stupid enough to try to break those laws. Employees can't keep a secret. And most of the claims costs are directly passed on to employers (group buyers) anyway, so the major health insurance companies have little direct incentive to deny coverage; with minimum limits on the medical loss ratio it's rather the opposite.
Companies can just not hire you for "culture fit" or something else based on leaked data about your health problems in order to keep their premium payouts low or just to avoid hiring certain types of people (fill in your blank here).