Taste aversion is an unusual form of classical conditioning because:

1. Ordinary acquisition needs repeated pairings and a brief gap between the conditioned and unconditioned stimulus, sometimes as short as five seconds. 2. Taste aversion breaks both of these rules at once. 3. Several hours may pass between eating the food, which is the conditioned stimulus, and the nausea, which is the unconditioned stimulus. 4. A single negative experience is enough to produce a lasting aversion, as in the chapter's curry example where the illness was actually caused by intestinal flu. 5. This is thought to be an evolutionary adaptation helping organisms avoid harmful foods, which is why it is so fast and so durable rather than quickly reversed. _Source: OpenStax Psychology (1st ed., CC BY 4.0), Ch 6 "Learning", §6.2 Classical Conditioning_