The symptoms of infectious mononucleosis (known as "mono") are nonspecific at the beginning. Lack of energy and fatigue are common initial complaints. These can be accompanied by a loss of appetite and chills. These early symptoms may last for a few days before more severe symptoms begin.

The characteristic symptoms of mono include a painful sore throat. Fever can occur and may persist for several days. It is common to experience swollen lymph nodes in the neck. Along with the painful sore throat, the throat may be reddened, and the tonsils may appear swollen with a whitish coating on the surface. Often, the sore throat is what prompts affected people to seek medical care.

Enlargement of the spleen occurs in up to half of people with mono. Enlargement of the liver can also occur, along with inflammation that raises blood levels of liver enzymes. Less commonly, a red rash may appear all over the body. Some people notice a swelling of the upper eyelids early in the course of the illness.

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