A surefire way to set off a panic in the office is to tell people that you have an insidiously contagious condition such as pink eye or head lice. Of course, we aren't suggesting that you spread said pestilence and carry out the worst possible April Fools' Day prank in history. It's all about subtle trickery.
Bring a bottle of lice powder or pink-eye medicated drops to work and casually leave it in the bathroom when no one else is around. The key for this one is stealth; don't let anyone see you leave the meds behind in the bathroom. To add more fuel to the fire, scratch your head or rub your eyes whenever you're around someone. Soon enough, co-workers will be abuzz with paranoid theories about who infected the workplace.