If you are sick you must inform your employer immediately and have your doctor issue you a Arbeitsunfähigkeits-Bescheinigung [statement of fitness for work]. This is a medical confirmation that you are unable to work due to illness.
More details
The Arbeitsunfähigkeits-Bescheinigung [statement of fitness for work] contains the expected duration of the incapacity to work, diagnoses and other information. The certificate itself is usually transmitted digitally to us from your doctor`s practice. In this case your doctor will only provides you a copy for your records.