Over here in germany the company can ask for a medical certificate, however in our CLA's as with many other companies it only has to be produced after more than 3 days of sickness. Sending it in by mail is sufficient. If one becomes sick during his vacation he can also visit any MD and get the certificate, call in sick to the company and get his vacation days refunded to be taken later during the year. During sick leave one is paid the normal rate of base pay and the average of the last 12 months of any additional pay, up to six weeks by law, in my outfit up to 9 months depending on time with the company.
So for any short-term sickness up to three days just calling in is sufficient.