Can I be fired if I get sick during my probation period?

The most important thing: during your probationary period you're still protected by sick leave rules, even though your employer has more freedom in deciding not to confirm you. You can't be fired just for getting sick, but illness can affect your overall evaluation during probation.

In practice, your employer still has to follow proper procedures: if you're sick, you report the absence and provide a medical certificate like anyone else would. What changes is that during those first months he has more leeway to not renew your contract when it expires, and he could use the frequency of your absences as one of the factors in the final decision. But he can't fire you the day you report being sick by saying "you're sick, get out": there would still need to be a legitimate reason for the termination.

If the illness is prolonged and heavy on productivity, it's a good idea to document everything (medical certificates, communications with your employer) and if you notice red flags - like pressure to resign, hostile behavior after your absence - keep your eyes open. If you really feel like they're looking for an excuse to push you out, better to get a sense of the situation from someone who knows your contract well.

Technically, during the probation period the employer has more leeway, but they can't fire you directly because of the illness itself - that would be unlawful. The real risk is that they won't confirm you at the end of the period, and unfortunately they have a free hand there without having to give specific reasons. The trap that nobody really emphasizes is that if you're absent too often or for too long, the employer could claim you're not suitable for the role, and technically this reasoning has nothing to do with the illness but in practice it penalizes you just the same.