partitioned into primary and secondary infection. These values are unaffected by host demography. However, the endemic level of infection is highly sensitive to changes in R 0 when R 0 > 1. If R 0 is increased by shortening the infectious period, then the endemic level of infection increases monotonically. However, if increases in transmission rates or decreases in the decay of free-living inoculum increase R 0 , then the endemic level of infection first increases (for 1 < R 0 < 2) but then decreases (for R 0 > 2). Consequently, increasing the intensity of control can result in more endemic infection. Identification of a Highly Conserved H1 Subtype-Specific Epitope with Diagnostic Potential in the Hemagglutinin Protein of Influenza A Virus Subtype specificity of influenza A virus (IAV) is determined by its two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). For HA, 16 distinct subtypes (H1–H16) exist, while nine exist for NA. The epidemic strains of H1N1 IAV change frequently and cause annual seasonal epidemics as well as occasional pandemics, such as the notorious 1918 influenza pandemic. The recent introduction of pandemic A/H1N1 IAV (H1N1pdm virus) into humans re-emphasizes the public health concern about H1N1 IAV. Several studies have identified conserved epitopes within specific HA subtypes that can be used for diagnostics. However, immune specific epitopes in H1N1 IAV have not been completely assessed. In this study, linear epitopes on the H1N1pdm viral HA protein were identified by peptide scanning using libraries of overlapping peptides against convalescent sera from H1N1pdm patients. One epitope, P5 (aa 58–72) was found to be immunodominant in patients and to evoke high titer antibodies in mice. Multiple sequence alignments and in silico coverage analysis showed that this epitope is highly conserved in influenza H1 HA [with a coverage of 91.6% (9,860/10,767)] and almost completely absent in other subtypes [with a coverage of 3.3% (792/23,895)]. This previously unidentified linear epitope is located outside the five well-recognized antigenic sites in HA. A peptide ELISA method based on this epitope was developed and showed high correlation (χ(2) = 51.81, P<0.01, Pearson correlation coefficient R = 0.741) with a hemagglutination inhibition test. The highly conserved H1 subtype-specific immunodominant epitope may form the basis for developing novel assays for sero-diagnosis and active surveillance against H1N1 IAVs. Influenza A viruses (IAVs), members of the Orthomyxoviridae family, are highly contagious to a variety of avian and mammalian species. IAVs cause seasonal influenza epidemics annually and recurring pandemics with severe consequences for public health and global economy [1, 2] . At least three IAV-pandemics emerged in the last century (1918 A/H1N1, 1957 A/H2N2, and 1968 A/H3N2). The 1918 Spanish flu was the most serious influenza pandemic that killed over 50 million people worldwide [3] . The latter two pandemics, although mild compared to the 1918 incidence, resulted in significant mortality, with close to 2 million and 1 million deaths, respectively [4] . The latest pandemic influenza, and newest global health challenge, occurred in 2009 due to the emergence of an A/ H1N1 pandemic IAV (H1N1pdm virus). The H1N1pdm virus has been detected in more than 214 countries and territories and has caused 18,389 deaths as of July 30, 2010 [5] .