We recommend informing employees that they may have been exposed to chicken pox, but would not reveal the name of the employee who has the condition. We would also limit the announcement to those employees who have a need to know, i.e., those who may have come into contact with the infected employee or those you know to be immunocompromised.
You do not have to allow this employee to come to work while infected. However, if they say they are no longer contagious and would like to return, you can require the employee to provide a doctor’s note to that effect. Once you have the note, you can decide how best to handle the situation based on what the doctor recommends.