It is not mandatory to tell your employer. It is left up to the individual. If one needs reasonable accommodation, usually medical records are required as proof of this need. You are not required to inform your employer or tell your co-workers about your positive HIV status. If needed, reasonable accommodations must be made.
Some people have no problem returning to work.
If not, your employer may contact the CDC for information regarding this issue. Some local and state laws may also provide protection from discrimination for people with disabilities. Contact your Human Resources Department, EEOC, or legal Counsel. Some examples of accomodations may include a flexible work schedule to allow for doctors' appointments and counseling, unpaid leave, job restructuring, modification of equipment and adjusting training materials or policies. It depends on the insurace plan for employees and their dependents.