Int. J. Environ. Res. Public Health 2010, 7 952 Now let’s return to the influenza epidemic process. Founder of HelioBiology, A. Chizhevsky, performed comparative retrospective analysis of flu epidemics with respect to solar activity cycles and came to following conclusions [1]: (a) Occurrence of major influenza epidemics shows definite periodicity with an average period of 11, 3 years, equal to the period of Solar activity fluctuations; (b) As a rule, significant influenza epidemics do not occur in years of minimum solar activity; (c) Most major influenza epidemics occurred in time interval starting 2–3 years before and ending 2–3 years after solar activity maxima. Several other authors also noticed correlation between cycles of solar activity and major influenza and other epidemics on the Earth [14-17,86]. Solar activity was estimated using reliable records of sunspot number dating back to at least 1700 and Solar Flux measurements started later on. Data about influenza pandemics in human history are available in several reviews: [87-89], and others. Good visual and statistical materials concerning possible correlation between level of solar activity and influenza pandemics are contained in the work Tapping et al. [86]. The authors demonstrated that influenza pandemics often develop in periods of solar activity maximum (Figure 9). But there are some exceptions, such as the pandemic of 1977, which occurred close to activity minimum. Figure 10 presents a summarized correlation picture for all studied period (about 300 years), showing existence of rather strong interrelation between solar activity and activity of influenza epidemic process. Figure 9. The 1946, 1957, 1968 and 1977 pandemics (shown as spikes) on a plot of the 10.7 cm Solar Flux index. The flux values prior to 1947 (shown dotted) are estimated from sunspot data. (From [86], reproduced by the kind permission of Dr. Tapping). For the sake of objectivity, it should be noted that many authors negate existence of predictable pattern of pandemic periodicity [90]. But the main reason of the “unpredictable periodicity pattern” can be polygenic nature of the epidemic process, i.e., existence of complicated interplay of numerous epidemic-predisposing and epidemic-preventing factors, including those of anthropogenic nature (migration, vaccination, etc.). These factors may mask regulatory influence of the solar activity cycles.