In this paper, we study pandemic viruses that spread during the period (1759-2020). Our findings and results include the following: (1) the pandemic severity level has a negative correlation with the strength of solar activity; (2) pandemic viruses have three types according to their compatibility with solar activity association. Most of them spread through the quiet Sun, where viruses survive better in cold and rainy weather; (3) the emergence of new strains of influenza viruses was manifested in two ways. First, the annual epidemics due to antigenic drift. Second, pandemics recur at 1-12 solar cycles (~11-120 years) due to new virus subtypes resulting from virus reassortment; (4) pandemic viruses have two groups according to their recurring period: first, recurring in nine solar cycles; second, recurring in twelve solar cycles. Furthermore, we reassortment pandemic viruses from their previous spread in the same periodic classification. Moreover, we derivative a periodicity formula of each subtype of pandemic viruses as spread date.