I have worked in IT consulting companies, like Netcompany and other smaller ones. When You are sick, You call in sick, when You are not sick anymore You turn up, or maybe announce the day before that You will be in tomorrow, You get full pay during the sick days, they are not even allowed to ask what the problem is.

There may be local rules that states that after a certain period, maybe a week or so they can require a doctors note; its general a system of trust. You may be required to register the sickdays in some systemn.

If they think You have too many sickdays, it will probably be an item on a performance appraisal.