It’s common knowledge that people are usually diagnosed with mononucleosis (mono) during adolescence. This is the reason it’s often referred to as the “kissing disease.” What lots of people don’t know, however, is that about 20 percent of children get mono before age ten. 

The explanation for this fact lies in the way the human body responds to infections. When a person comes down with an illness, his immune system produces antibodies to help kill the invading microorganism. The first antibodies the body produces are in the IgM class. These antibodies are present for about three months after which they disappear from the circulation.

A month or so after the body starts producing IgM antibodies, it makes IgG antibodies to the same viral or bacterial agent. However, IgG antibodies are produced continuously and will remain in a person’s bloodstream for life. The reason you don’t get the same infection more than once is because IgG antibodies are always around, and they prevent the infection if you are exposed to the virus or bacteria in the future. This is what it means to be “immune” to something.

Antibody production also explains how immunizations work. When you get a vaccine, your body is injected with a tiny amount of a killed or weakened biologic agent. Your body then makes antibodies to that microorganism so if you get exposed to the germ at a later time, you won’t get sick or you will have a milder form of the illness. In most cases, the body’s immune system is superior to vaccinations in terms of conferring future immunity. That’s why vaccines are not 100 percent effective. A notable exception is whopping cough, which is better prevented by vaccines than natural infection.

Okay, back to mononucleosis. When I draw blood to check for mono, I always test for IgM and IgG antibodies to the virus. If the patient’s IgM antibodies are positive, that means he currently has mono. However, if the IgM antibodies are negative, but the IgG antibodies are positive, that means he had mono at some point in the past, but does not have it now, i.e., the current infection is due to another virus. If the patient has a negative test for IgM and IgG antibodies that means he doesn’t have mono now and did not have it in the past either.

Parents almost never recall a preteen child having had mononucleosis because this age group does not get the classic mono symptoms: fever, swollen lymph nodes, horrible sore throat and fatigue. Instead, they usually have a fever that lasts for four or five days.