Our bodies work like computers, in the way that they are being programmed like computers. I have bad experience from fisherman shots, this problem occurred several years ago but I still have a problem with spearmint. I feel sick even if its only chewing gum with spearmint taste. This is because my brain has registered the mint taste as something bad and as a consequence of eating it I feel sick. The same goes for all other experiences, it can for example be someone you hated as a kid who always wore a lot of perfume and then as a grown up you can have a serious problem with perfume. Although not a real allergy, the feeling is only an association with a bad experience in the past.
There is a therapy method to cure this that is called NLP = Neuro-linguistic programming. This method tries to reprogram your body, its very interesting. Read more on wikipedia for more info!