No, you do not have to do so by law. Your employer is not allowed to know why you are ill or to ask about the cause, i.e. what sort of illness it is. However, your employer is entitled to ask when you expect to be able to work again. If you are ill for more than a week, your employer will report it to the company doctor, who looks at what sort of illness it is, how long it will last and whether you might be able to do adjusted work. Together with you and your employer, the company doctor makes a plan for your recovery and how you can return to work. Your manager will expect that you do everything possible to recover as quickly as possible. That is in your own interests as well.