Not everyone infected with the virus that causes mono (Epstein-Barr virus, or EBV) has symptoms. This is especially true for young children, who may have a fever but no other symptoms. People ages 15 to 24 are most likely to have obvious symptoms.1
The most common symptoms of mono are:
These symptoms usually get better in about 1 or 2 months.
You can get a rash if you take the antibiotics amoxicillin or ampicillin when you have mono. These antibiotics are often prescribed for other causes of sore throat, such as strep throat, and might be prescribed for you before the doctor knows you have mono. The rash is not an allergic reaction.2
Mono may cause your spleen to swell to 2 or 3 times its normal size. An enlarged spleen occurs in up to 75 out of 100 people who have mono.3 A blow to the abdomen can cause an enlarged spleen to rupture. To reduce this risk, avoid heavy lifting and contact sports for several weeks after you become ill with mono or until your doctor says it is safe. In very rare cases, the spleen may rupture on its own.
Symptoms of mono can be more severe and last longer in people who have an impaired immune system or a rare genetic condition called X-linked lymphoproliferative syndrome.
The symptoms of infectious mononucleosis, such as a sore throat and fever, also are found in many other conditions.