My 17-year-old daughter has been diagnosed with mononucleosis for the second time. Her symptoms are somewhat different this time... much more fatigue and sweating. Can you really get mono more than once? Is there anything different about a second infection? Could she get it again after this?

P.M.

Infectious mononucleosis, or mono, appears most commonly in adolescents and young adults, such as your daughter. This disease is caused by infection with the Epstein-Barr virus (EBV), which generally spreads through close contact with saliva. Most people with mono do not recall being exposed to anyone with the infection. One reason for this is that some individuals can spread the infection without having any symptoms at all. In fact, most cases of EBV infection are asymptomatic (without symptoms), especially in children, and are not diagnosed as mono.

When symptoms do occur, patients have a group of three classic symptoms: sore throat, fever and lymph node swelling. These symptoms usually last less than two weeks, though fatigue can last for more than a month. Complications may include blood abnormalities and rupture of the spleen, although in most cases, the infection resolves on its own without any additional problems. After infection, the virus remains in the body, but only rarely causes problems again. The patient is then immune to future infection, so it is unlikely that your daughter really had mono due to EBV more than once.

Some medical scientists have proposed that EBV is a cause of chronic fatigue syndrome. In this illness, the patient complains of a relatively abrupt onset of overwhelming fatigue, worsened by activity and not relieved by rest or sleep. It is often preceded by a typical respiratory infection, and sometimes by a case of mono. However, various studies have failed to show that EBV is an important cause of chronic fatigue syndrome. There are rare patients who have persistent disease caused by EBV. But because such cases have obvious signs (such as liver damage, fever and weight loss) rather than just fatigue and sweating, I doubt your daughter has this problem.

What could your daughter have? Well, it is possible that her symptoms have been caused by two different infections. One of the infections could have been EBV, while the other could have been something that causes similar symptoms. Possibilities include cytomegalovirus, toxoplasmosis, hepatitis B and even HIV, the virus that causes AIDS. EBV infection and each of the other possible diagnoses can be confirmed by blood testing. Some of the rapid tests for EBV can be in error, though, falsely indicating that your daughter has had the infection more than once. She also could have a variety of other infections that cause fatigue and sweats, including influenza, parainfluenza and even viruses of the common cold. If any of these infections is responsible for her illness, however, I would expect her to have other symptoms, especially coughing, a runny nose or other manifestations of a respiratory tract infection.

Your daughter also may have a noninfectious illness masquerading as mono. True mono usually causes fever, and you do not mention fever at all. Fatigue can also be seen in thyroid disease, depression and, of course, chronic fatigue syndrome. You do not describe how long your daughter's illness has lasted. If it has been less than two weeks and she is not having fevers or significant other problems, then she may be able to simply wait for the illness to resolve. If it has been considerably longer than that, or if she has fever or other signs of infection, she should probably have the specific antibody tests for mono and at least some of the other infections I mentioned.