I trust my manager to not be a dick when I'm sick. I also believe he is an empathetic person who cares about me and wants to know what's up. I understand he has a list of tasks that need to be completed by his small team. So I'm happy to let him know whether I expect to be out for a day or a week.

If I didn't have such a relation with my manager I'd be looking for a job. If he doesn't look out for me when I'm sick he will probably not look out for me professionally either.