Texts from Egypt's New Kingdom period suggest that even when workers were too sick to build the royal tombs, they were still paid. The pharaohs apparently calculated that allowing for health needs leads to better work. And that's a society that literally treated many workers as slaves.
Part of the problem with the conventional concept of sick days is that you generally need to wait until you require medication, go to an emergency room or have a breakdown to take one.
Some 5,000 years later, we need to modernize how we support worker health — though companies that have yet to offer even basic paid sick leave should finally catch up to the ancient Egyptians. Instead of merely continuing to compensate workers once they are ill, we need to rethink time off to keep employees healthy.