I'm not saying periods are not serious, just that regardless of it being cancer or tragedy, someone will be covering the responsibilities of the absent person.

Some jobs don't have that much responsibility or the work style is a periodic deliverable, which makes this work.

But if you set up a new one-man business and needed a hire someone to deliver fresh flowers, you won't choose an interviewee who says they have cancer so they need to take a medical leave every month for a week, and they expect that to be paid.