Viruses are generally thought of in the context of making people sick, but there are actually some that are so ubiquitous that they are barely even noticed in most adults. Human cytomegalovirus is an example. It is extremely transmissible, and as a result it infects 80 percent of the adult population—if "infects" is even the word for something that is easily kept in check by the immune system in most adults.
Occasionally, however, cytomegalovirus can make someone sick. This is most common in people with weakened immune systems, such as after bone marrow or organ transplants or as a result of AIDS. Cytomegalovirus is commonly transmitted to fetuses from their mothers if the mothers became infected during pregnancy—in fact, it is the virus most frequently passed from a pregnant woman to her fetus. It can cause serious complications in these cases, with babies being born with jaundice, low birth weight, enlarged spleen or liver, or a rash. Typically, however, these children grow up none the worse for wear if they are carefully monitored, though sometimes it can result in birth defects that result in life-long health problems.
Cytomegalovirus in adults can have more long-lasting effects. Symptoms in adults include changes in behavior, blindness, coma, diarrhea, encephalitis, hepatitis, pneumonia, seizures, and digestive ulcers which sometimes bleed. If it’s ignored, the disease can even be fatal—generally speaking, only people who are showing symptoms would benefit from treatment, but people who are showing symptoms absolutely need to get themselves treated. There appears to be a link between cytomegalovirus and brain cancer, where the tumor activates the virus if its dormant and virus helps the tumor to thrive.
Infants with cytomegalovirus infection are generally treated for symptoms, such as pneumonia. Anti-viral medications can control the virus in children and adults but they cannot eradicate it entirely. People who have had bone marrow transplants are prone to opportunistic cytomegalovirus disease when their weakened immune systems can no longer keep the virus at bay, but some antivirals have been found to be particularly effective at preventing this from occurring.