In the 1920s, John Watson, the founder of behaviorism, conditioned an 11-month-old baby known as "Little Albert" to be scared of a rabbit and a rat. He did this by making disturbing noises when the baby was shown either of these animals.
We use classical conditioning in dog training all the time. In our puppy socialization classes, we play the sound of a truck backfiring and then give a treat. Eventually the loud noise won't be so scary to a dog. This is learning by association, which is also called classical conditioning. And it works on humans, too. In the above video, some college kids explain his undertaking in a way that made me laugh, and might make me forever associate good things with the sight of grown men in diapers.