Now You See It - Now You Don't

Will someone tell me why - when it's so easy to catch a cold, a bus, train or plane, small ones misbehaving, cheating spouses, lying politicians, robbers and rogues, someone staring, a clip across the ear, a cold sore, a cricket ball, a good night's sleep, a catch-you-later promise, inappropriate infections, computer viruses, hives, hair lice and a tiger by the tail - it's impossible to catch FRUIT FLIES!