Can I be fired for missing work due to illness without a medical certificate?

I had a colleague who missed a few days without a medical certificate and got a warning, but wasn't fired on the spot. Legally, the company can't fire you just for that if you were actually sick, but they can demand proof - and if it becomes a pattern of absences without justification, that's when things get messy. The best thing to do is try to get a medical certificate even if it's afterward, by talking to a doctor, or let your boss know as soon as possible so it doesn't look weird. If HR gets really upset about it, it's worth reaching out to a union or someone who knows labor law to give you better guidance.