Infectious mononucleosis (called "mono" for short) is caused by the Epstein-Barr virus (EBV), a virus that affects nearly everybody at some point. The disease is sometimes known as glandular fever, because it causes lymph glands to swell up. It's also called the "kissing disease" because kissing is a common method of transmission.
Before 5 years of age, 50% of individuals are infected by EBV. By 40 years of age, 90% of individuals are infected. Once you've caught the virus, it never really goes away. However, it can only cause symptoms in healthy people when it's still new in the body. After that it's kept in check by the immune system, though never completely eradicated.
Of course, nowhere near 90% of the population has suffered from mononucleosis. Half of all people are infected with EBV before age 5, and this age group hardly ever has symptoms beyond occasional mild tonsillitis. Unless their immune system collapses, they will never have mononucleosis. In the 5 to 30 years of age group, most studies have found that about 10% of people with EBV get the symptoms of mononucleosis. The exception is college students, who have rates of around 1 in every 2 cases of infection, which is several times higher than non-college students of the same age.
There are rare cases of mononucleosis occurring in older people, but usually the ones affected have weakened immune systems. Older people will usually have been infected much earlier in their lives. Such cases tend to be severe but not deadly.
In recent years, a link has been postulated between EBV and myalgic encephalomyelitis/chronic fatigue syndrome, but the latest studies seem to refute that. There is, however, a longstanding link with Burkitt's lymphoma, a cancer that occurs mostly in central Africa. EBV is also linked to other cancers such as Hodgkin's disease. EBV is one of very few viruses known to be capable of causing cancer.
There's also a weak link with nose and throat cancer. The small minority of people who don't carry EBV have a lower-than-average chance of developing this cancer.