That seems to be a decision which must be discussed between a doctor and patient. It becomes more evident over time in that patients who loose CD4 cells are generally placed on more aggressive therapy than people with stronger immune systems. I don't know if there is any legal obligation to tell your employer. I think you're actually legally protected from the disclosure of that information. You would have to consult with an attorney.
Different states have different laws.
Generally HIV care is so expensive that most can't pay for it out of their own pocket. So, out of necessity they have to rely on health care insurance. The problem of insurance is quite real, many people with individual policies have their premiums doubled every 6 months until it becomes financially impossible to pay. In essence they are squeezed off of insurance and then become forced into state programs for health care which are paid for through taxation of all of us. Generally larger group policies offer or afford more protection to the individual, although I have seen situations where the insurers facing huge costs from HIV care have withdrawn insurance from the entire state of California.