The sensations of taste and smell are related, so many disorders of the sense of taste are associated with a decreased sense of smell. These taste disorders can range from obstructions in or damage to the nose to damage to the brain and nervous system in general. The most common pure taste disorder is a phantom taste sensation; this is the perception of a "bad taste in the mouth" that does not go away. The decreased ability to taste certain types of foods is known medically as hypogeusia; the absence of taste entirely is termed ageusia. Dysgeusia refers to the presence of a metallic, rancid, or foul taste in the mouth. Taking certain medications can also interfere with the ability to taste. Some loss of taste sensation also occurs during the normal aging process, so elderly people may complain of decreased ability to taste foods. Sometimes, having a cold or upper respiratory infection can result in a decrease in taste sensation.