Mononucleosis, often referred to as "mono," is a very common viral illness. About 90% of people over 35 have been infected with it, probably during early childhood, and have antibodies to the virus in their blood.
When mono strikes young children, the illness is usually so mild that it's not noticed or passes as a common cold. When it occurs during adolescence or adulthood, however, the disease can be much more serious.
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Mono usually comes on over a few days. It begins with flu-like symptoms -- fever, headache, and general malaise and lethargy. After a few days, the lymph glands begin to swell, although this symptom is not noticeable in everyone. Swollen glands of the neck are especially typical of mono. Most people develop a sore throat, which can be very severe, with inflamed tonsils. A fever -- usually no higher than 104 degrees -- also can develop and may last up to three weeks.
Some people, particularly those who take the antibiotic amoxicillin, may develop a red rash all over the body. Others may notice red spots or darkened areas in the mouth that look like bruises. In about half of all cases, the spleen becomes enlarged, which may cause an area in the upper-left abdomen to become tender to the touch.
In most cases, mono mildly affects the liver. Only a few individuals with mono develop jaundice, a yellowing of the skin and eyes caused by an increase of liver protein in the blood. In rare cases of mono, the liver fails.
Other major complications that can develop from mono include rupturing of the spleen, low blood platelet count, swelling of the lining of the brain or spinal cord, or inflammation of the brain itself. These complications are extremely rare.
Most people who come down with mono feel much better within two or three weeks, although fatigue may last for two months or longer. Some people feel the disease lingers for a year or so.
In the past, research suggested that the virus causing mono might be linked to a persistent, debilitating illness known as systemic exertion in tolerance disease. SEID was formerly called chronic fatigue syndrome and can last for years. Most recent research has shown no such link, however, and the cause of SEID remains unknown.