A positive IgM antibody test in someone who has not been recently vaccinated means it is likely that the person has the current measles or mumps infection. Positive IgM and IgG antibody tests or a fourfold increase in IgG levels (titers) in blood samples collected several days apart mean it is likely that the person has a current or had the recent measles or mumps infection. When measles or mumps IgG antibodies are present in a person who has been vaccinated and/or is not currently ill, then that person is protected against infection (immune). If a person does not have measles or mumps IgG antibodies, then that person is not considered immune to the viruses.