Disgust is a type of aversion that involves withdrawing from a person or object with strong expressions of revulsion whether real or pretended. It is one of the basic emotions and is typically associated with things that are regarded as unclean, inedible, infectious, gory or otherwise offensive. In The Expression of the Emotions in Man and Animals, Charles Darwin wrote that disgust refers to something revolting. Disgust is experienced primarily in relation to the sense of taste (either perceived or imagined), and secondarily to anything which causes a similar feeling by sense of smell, touch, or vision. Musically sensitive people may even be disgusted by the cacophony of inharmonious sounds. Fear of contamination, by insects, waste products or any kind of corruption, may inspire disgust. In this case, disgust arises from a process of inference from perceptual experience. For example, the understanding that insects have, in the past, caused pestilence my lead to a present-moment extrapolation that certain other insects, however innocuous, are disgusting because they are causing, or could cause, disease. Disgust is one of the basic emotions of Robert Plutchik's theory of emotions. It invokes a characteristic facial expression, one of Paul Ekman's six universal facial expressions of emotion. Unlike the emotions of fear, anger, and sadness, disgust is associated with a decrease in heart rate. It is necessary to resist the temptation to universalize in dealing with such complex states of mind and emotions as disgust. People in many professions, such as medical care, police work, fire fighting and the military learn to repress their disgust responses and may even lose the capacity to experience disgust altogether.