Influenza produces upper and lower respiratory tract infection. There are 3 major types or subgroups: A, B, and C. Antigenic mutations occur frequently in type A, permitting reinfection; but are much less common in types B and C. The best laboratory test is culture of nasopharyngeal secretions, especially by the newer and much more rapid (2-4 days) shell-vial culture technique. Antibody detection is available by CF, IFA, and ELISA. IgM and IgG can be assayed. Acute and convalescent specimens are needed unless the initial CF or IgM antibody response is positive in high titer (both CF and IgM antibody disappear in a few weeks). A rapid membrane-filtration cartridge EIA method is commercially available for influenza A virus, which was found to have 90% sensitivity (compared to culture) using nasopharyngeal washings and 39% sensitivity using pharyngeal gargle samples.