In view of presumed non-pathogenicity, tumor suppressive properties, and site-specific integration of the viral genome the human parvovirus, adeno-associated virus (AAV) has gained great interest as a gene transduction vector. Data on the seroprevalence of antibodies to AAV vary between reports, probably due to the different serological methods used. In order to understand better the immune response to AAV during natural infection, sera from different age groups and various geographical regions were compared for AAV antibodies using an ELISA. The data show that the prevalence of antibodies to AAV is similar in Europe (Germany, France, and Switzerland), Brazil, and Japan, indicating worldwide infection. It was confirmed that infection takes place during childhood. However, declining seropositivity thereafter and a second increase of seropositivity after 30 years of age suggests reinfection or reactivation of latent virus in particular as the prevalence of IgM antibodies in adults is relatively high. Furthermore, pregnant women were found to be significantly more frequently seropositive than non-pregnant controls, hinting at a reactivation of persistent AAV (up to 80% of women carry AAV in genital tissue) in specific hormonal conditions, e.g., pregnancy. Cross-reaction of serum antibodies with the different AAV types (defined by complement fixation) was observed by ELISA and neutralization tests confirming earlier results. The results suggest an unstable AAV antibody response allowing lifelong reinfection or reactivation of persisting virus possibly due to partial immunotolerance after an infection in utero, at delivery or during early infancy.