The detection of agent-specific IgM antibodies in patient serum is generally considered to confirm an acute infection. However, IgM antibodies can be found only temporarily during the course of the disease and do not allow identification of all acute infections. In other cases, the IgM persists for months or years after recovery. If the IgM investigation was not helpful, a modern diagnostic laboratory nowadays is also able to serologically diagnose a fresh infection by determining the avidity of the specific IgG: At first contact with an unknown infectious agent the organism starts to produce antibodies with a weak affinity. During the course of the disease, however, the adaptation of the B-lymphocytes to the agent becomes more accurate, inceasing the initially low avidity of the specific IgG by several powers. The determination of high-avidity IgG antibodies at first testing is therefore a good indication that the actual infection took place some time ago, whereas the presence of low-avidity IgG confirms a fresh infection.