Generally it is compulsory in Germany to have a health insurance, regardless of the occupational or person group:
From your first working day onwards you need to have a health insurance. In Germany der is a distinction between statutory health insurances and private health insurance. You can be insured in a private health insurance only, if you have a yearly gross income of more than 53,500 €. However, statutory health insurances are obliged to insure you. The insurance premium is calculated for everyone individually proportionally to your salary. Half of it will be paid by your employer, the other half will be directly subtracted from your salary (between 14% and 18% of your gross income). This premium includes your share on the nursing and social insurance already. This is standard in Germany and compulsory and binding for every employee.
Self-employed, students, retirees:
Those, which have never been member of a health insurance, will be insured in the system he is belonging to because of his job (retirees and students too). Self-employed and freelancers belong to the system of private health insurances.