Antibodies to common microbial antigens have been used to detect infection with different microbes for years. However, the presence of a single antibody serum specimen only tells you that the person was exposed to this microbe in the past. To diagnose an acute infection, one must have paired sera usually taken two weeks apart that demonstrate a significant rise in antibody titers in the second specimen compared to the first. One can also do these antibody tests in reverse. Normally, we are exposed to many microbial antigens during growth and development either as a result of exposure to a given microbe or after immunization with a given antigen (ie tetanus toxoid, pneumococcus polysaccharide, measles, or mumps viral antigen). Antibodies to the microbial products are usually found in normal individuals, but if they are not, one should suspect abnormalities of antibody production like those seen in immunodeficiency.