Publisher Summary Particles of influenza virus consist of at least three internal proteins (ribonucleoprotein, matrix protein and RNA-polymerase) contained within a lipid envelope, from which two distinct surface glycoprotein subunits, the hemagglutinin and neuraminidase, protrude as “spikes.” The genome of influenza viruses is segmented, single-stranded RNA which exists in 6 or 7 pieces. RNA isolated from the virus particles is not infectious, probably because it needs to be first transcribed by the virion polymerase into complementary messenger RNA. The segmented nature of the genome allows recombinant viruses (anti-genic hybrids) to be formed readily during mixed infections, and evidence has been obtained which suggests that the viruses responsible for the great human pandemics may be formed by genetic recombination between animal (or avian) influenza viruses and human strains. Such recombinants, having the hemagglutinin or neuraminidase subunits of the animal or avian virus, and the virulence for and the capacity to spread in man, would meet with little antibody resistance in the human population and pandemic influenza would result. Antigenic drift that occurs in influenza viruses between pandemics seems to result from the gradual changes in the amino acid sequence of the polypeptides of the hemagglutinin and neuraminidase subunits.