Seroepidemiologic Studies: EPIDEMIOLOGIC STUDIES based on the detection through serological testing of characteristic change in the serum level of specific ANTIBODIES. Latent subclinical infections and carrier states can thus be detected in addition to clinically overt cases.Antibodies, Viral: Immunoglobulins produced in response to VIRAL ANTIGENS.Antibodies, Bacterial: Immunoglobulins produced in a response to BACTERIAL ANTIGENS.Sparganosis: Infection of animals, including fish and man, with a developmental stage of Diphyllobothrium. This stage has recently been referred to as a plerocercoid but the name sparganum has persisted. Therefore, infection of fish or other animals with the plerocercoid larvae is sparganosis. Fish-eating mammals, including man, are the final hosts.Enzyme-Linked Immunosorbent Assay: An immunoassay utilizing an antibody labeled with an enzyme marker such as horseradish peroxidase. While either the enzyme or the antibody is bound to an immunosorbent substrate, they both retain their biologic activity; the change in enzyme activity as a result of the enzyme-antibody-antigen reaction is proportional to the concentration of the antigen and can be measured spectrophotometrically or with the naked eye. Many variations of the method have been developed.Chlamydophila pneumoniae: A species of CHLAMYDOPHILA that causes acute respiratory infection, especially atypical pneumonia, in humans, horses, and koalas.Prevalence: The total number of cases of a given disease in a specified population at a designated time. It is differentiated from INCIDENCE, which refers to the number of new cases in the population at a given time.Immunoglobulin G: The major immunoglobulin isotype class in normal human serum. There are several isotype subclasses of IgG, for example, IgG1, IgG2A, and IgG2B.