I have work later, but can't go in because I have a family thing to do, so I have to call out, what ridiculous disease should I tell my manager I am dying of?
The more visceral the better.
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Just keep it simple. Say you caught a cold. Don't want to say you have something that is supposed to take a long time to get over and show up to work the next day with no symptoms. Plus cold symptoms are easy to fake.