sick days

This is complicated but:

'Any employee who falls sick or who is the victim of an accident other than a work accident must inform the employer as soon as possible and, in principle, send a medical certificate within two working days. The employer is entitled to require that the employee is examined by a doctor designated by the employer, and in the event of disagreement between the employee's and the employer's doctor the labour court will appoint a third doctor to arbitrate.
The employer remains liable to pay the first seven days' wages for manual workers who have less than one-month's seniority and employees with less than one-month's seniority who have a trial clause in their contract or who are employed for a fixed term or a specific job for less than three months. For all other employees, the employer must pay the first 30 days of salary. For all employees, salary for the remaining period of sickness is paid by the social security system. If the employee falls sick again within 14 days of the end of the previous period of illness, the employer is not liable to the salary payments described above, unless the employee produces medical evidence that the second illness or accident is different from the first.
Any notice of termination served during the period of sickness will take effect only when the employee resumes work, and any notice previously served will be suspended during the period of illness.'
Also see: http://www.pwc.be/en/doing-business-guide/practical-information/labour-c...

The simple answer is that after two days off you need a doctors certificate, and at any time after that, you could be asked by your employer to have a medical by a second doctor nominated by them, to prove you are actually sick. If you refuse, or aren't actually sick, then they will either stop paying you, or fire you.
Assuming that you are sick, then your employer is responsible for paying your salary for the first 30 days. After that your mutuelle takes over.

There is no hard and fast rule that you can take x days a year off as an entitlement.
If you are ill you can be off as long as it takes for you to get better but, if your employer suspects that you are taking liberties, they can insist on a sick note for every day that you are off work.
As to 'without having any income affected', once your mutuelle takes over after 30 days, you will get about 90% of your basic salary and, should you become an 'invalid' - after 12 months - you will get about 70% and that will go on as long as you continue to be unfit for work or reach 65.