where Dr is the difference in growth rate between the incumbent and variant strains. Its equivalence to equation 4 is demonstrated in File S1, Section ii. It is evident that as N?large we recover equation 2, since Dr and DR 0 are positively correlated. We exploit this new theory of entropic epidemiology to explain detailed empirical observations regarding the emergence of less pathogenic strains of human influenza A virus, an issue that has remained elusive when viewed through the framework of classical epidemiological theory. Moreover, we discuss the current on-going swine 'flu (H1N1 2009) outbreak from the perspective of directionality theory, [18] [19] [20] [21] [22] .