You may, or may, not need the Yellow Fever card to get into certain African countries on any given day as Martinis at 8 rightly points out, but if you don't have it, you almost certainly won't get into your next non-yellow fever zone country, once the immigration officer spot's your previous entry/exit stamps. You'll almost certainly end up either in quarantine, if you're lucky, or back on the next return flight back to where you came from.

Get the Yellow Fever shot and card before you need it, it lasts ten years.

As someone who spends a lot of time in Africa, actually I'm here in Africa right now, I can't believe the FT threads of late with folks contemplating not taking anti-Malaria meds or travelling to Yellow Fever zones without the Yellow Fever vaccine. Next someone's going to ask if they should have unprotected sex with a complete stranger.

mpc1

Edited to Add: I know that no one in this thread has recommended/contemplated travelling without a Yellow Fever vaccine, but, the Malaria Thread leaves me just speachless