Seroconversion
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Seroconversion is the development of detectable specific antibodies to microorganisms in the serum as a result of infection or immunization. Serology (the testing for antibodies) is used to determine antibody positivity. Prior to seroconversion, the blood tests seronegative for the antibody; after seroconversion, the blood tests seropositive for the antibody.
The word is often used in reference to blood testing for anti-HIV antibodies.
Background
The immune system maintains an "immunological memory" against past pathogens to facilitate early detection and to confer protective immunity against a rechallenge. This explains why many childhood diseases never recur in adulthood (and when they do, generally point at immunosuppression).
After the initial phase of the infection IgM levels drop (and become undetectable) but IgG levels remain detectable. Upon reinfection, IgM levels rise again. IgG levels can increase slightly, but an elevated IgG titre cannot reveal if the infection is recent or currently active. Presence of IgM is a sign of current or recent infections. Presence of IgG suggests past infection or immunization.