Headcrab! Quickly, grab a crowbar!
This parasitic beings will crawl on four legs then jump (up to 10 feet) to latch themselves to the victim's head. Once attached they will use their beak to break into the skull and take control of the said human, turning them into zombies. Slip on the plushy Half-Life HeadCrab Hat for an easy costume with a huge impact. Sadly it won't turn you into a real life zombie.