Cytomegalovirus infections are common throughout the world. Certain populations, including pregnant women and their fetuses, immunosuppressed patients, and recipients of large amounts of transfused blood, are at increased risk. Although the majority of infections in all groups of patients are clinically inapparent, variable symptoms, including fever, rash, pneumonitis, and hepatitis, can occur. The infected host develops antibodies against CMF, but frequently, despite this appropriate immune response, infection becomes chronic with prolonged excretion of virus. In some instances, a latent infection, with disappearance of virus, develops and under a variety of circumstances, including immunosuppression, infection can later be reactivated with reappearance of viral excretion. The human consequences of latent infection with CMV are not yet fully appreciated, and future research on this virus with multifaceted potential will need to focus on this issue.