You need to inform your employer at the start of the first day you are unable to work due to sickness at the latest. You do not have to disclose what illness you have. If you can estimate how long you will be ill, however, you should let your employer know. You can inform the employer per phone, SMS or email.
What do I have to consider?
When you are sick and unable to work, you are entitled to rest at home. However, your employer has the right to demand proof; i.e. confirmation of illness/unfitness for work- which you can obtain from a doctor. Colloquially, such a document is known as "Krankschreibung" (sick note), but the official term for it is "Arbeitsunfähigkeitsbescheinigung " (certificate of unfitness for work) or -for short- AU.
Was muss ich wissen?
I am sick and cannot work. When do I have to inform my employer?
When do I need to obtain/present a sick note?
What does a sick note look like?
What happens if my employer doesn't trust a sick note?
Will I get paid while on sick leave?
Do I have to stay at home while on sick leave?
Can I stay at home when my child is sick?
Can I obtain a sick leave on the phone or online?
You must always inform your employer if you cannot get to work.