If you willfully refuse to self-disclose a highly contagious disease such as COVID-19, this could be in violation of health and safety legislation requiring employees to disclose workplace hazards as well as directives from public health authorities. Discipline could result due to the significant health and safety risk you could pose to other workers. Further, it is likely that an arbitrator would find some measure of discipline to be reasonable. The fact that you intentionally refused to disclose would be an aggravating factor.