How do blood tests show whether someone has an infection or has been vaccinated?

Blood tests can detect antibodies such as IgM and IgG, which rise at different times after exposure. A recent infection often produces an early IgM response, while IgG may indicate longer-term immunity from past infection or vaccination. Some tests also look for antigens or the pathogen’s genetic material to detect active infection directly. Interpretation depends on timing, symptoms and the specific disease being tested.