Do I have to go to the doctor immediately if I am ill?
If you are ill and unable to work, it does not automatically mean that you have to go to the doctor immediately. What is important is that you inform your employer immediately about your inability to work (see question: How do I correctly call in sick?). Then it always depends on whether your employment or collective agreement stipulates when you have to send your employer a certificate of incapacity to work at the latest. If nothing special is regulated, the statutory regulations apply: They stipulate that the employer must receive a sick note by the 4th calendar day at the latest. For example, this means that if you are only sick for 1 or 2 days and then return to work, you do not need a certificate of incapacity to work and thus do not need to see a doctor. However, many employees choose to go to the doctor anyway to make sure they are credible.
If your employment or collective agreement stipulates that you must submit a certificate of incapacity to work to your employer on the first day of illness, then you must of course go to the doctor on the first day you are ill.