Within the influenza A genus, there are several viral subtypes classified by the ability to produce an immune response of their surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). In contrast to other species, influenza A has relatively few subtypes capable of triggering outbreaks within the human population. Only viruses bearing both H1, H2, or H3 HA genes, and N1 or N2 NA genes have circulated among the human population throughout the 20th century. The years 1918, 1957, and 1968 were all marked by extremely high rates of morbidity and mortality related to influenza. These were also the years in which novel subtypes of influenza had begun to circulate among the population. Because the human immune system had not yet created antibodies for these viruses, the population easily succumbed to a flu outbreak which consequently left thousands dead or impaired. It has been estimated that the next flu pandemic could cause upwards of 200,000 deaths and possibly over 700,000 hospitalizations.