A conditioned taste aversion can occur when eating a substance is followed by illness. For example, if you ate a taco for lunch and then became ill, you might avoid eating tacos in the future, even if the food you ate had no relationship to your illness.
Conditioned taste aversions can develop even when there is a long delay between the neutral stimulus (eating the food) and the unconditioned stimulus (feeling sick).
In classical conditioning, conditioned food aversions are examples of single-trial learning. It requires only one pairing of the previously neutral stimulus and the unconditioned stimulus to establish and automatic response.