Sunspots And Flu
The last six sunspot peaks have coincided with flu pandemics. During the sunspot maxima of 1947, 1957, and 1968, the influenza-A virus underwent anti genic shifts that allowed the virus to bypass the immunity built up in the populace. In 1937, a pandemic occurred but no genetic change was detected, although one might have gone unnoticed. The deadly worldwide 1918-1919 epidemic transpired just after the 1917 sunspot peak and before the discovery of the flu virus. The sunspot maximum of 1928 may have signaled a major shift from the virus causing the 1918-1919 pandemic to the type now afflicting us.
(Hope-Simpson, R.E.; "Sunspots and Flu: A Correlation," Nature, 275:86, 1978.)
Reference. The curious phenomena of epidemics are cataloged at BHH6 and BHH7 in Biological Anomalies: Humans II. This Catalog is described here.