About 90 percent of people over age 35 have antibodies to mono in their blood, which means that they have been infected with it, probably during early childhood. When mono strikes young children, the illness is usually so mild that it passes as a common or the flu. When it occurs during adolescence or adulthood, however, the disease can be much more serious. Mono comes on gradually. Swollen glands in the back of the neck are especially typical of mono.
Most people develop a sore throat, which can be very severe, with inflamed tonsils.
About 10 percent of people with mono develop a generalized red rash all over the body. Some people may notice red spots or darkened areas in the mouth, especially on the palate, that resemble bruises. In about half of all cases, the spleen may also enlarge, causing an area in the upper left abdomen to become tender to the touch. In 95 percent of cases, the illness affects the liver. However, only about 5 percent of individuals with mono develop jaundice, a yellowing of the skin and eyes caused by an increase of bile pigment in the blood.