IgG positivity to cytomegalovirus is usually due to infection with cytomegalovirus. This virus is widespread among humans, and in an immunocompetent person, the infection is likely to be followed by no obvious symptoms, or just one mild fever, similar to a cold. Therefore, a positive IgG antibody to cytomegalovirus is a previous infection with this virus. Having been infected does not usually cause particularly noticeable abnormalities, and long-term positive IgG antibodies indicate that one has developed some immunity to this virus. Cytomegalovirus infections can be harmful in two ways: one is HIV infection, where the immune system is very weak, which can lead to more serious infections. The other is the first infection with cytomegalovirus after pregnancy, which may cause malformation of the fetus. If a person has been infected with cytomegalovirus before pregnancy, and is only positive for cytomegalovirus IgG antibodies, then infection with cytomegalovirus will have a significantly lesser effect on the child.