” solid link between GSM(Grand Solar Minimum)s and major plagues that originate in China.”
It had nothing to do with the magnetosphere. It was the weather. The Grand Solar Minimum cause long cold winters and years with no summers.
Flu and Virus happen mostly during the cold seasons.
I think this long cold weather seasons allow the virus like the Black Death to travel great distances. The virus/plagues would follow the trade routes when trade open up between Europe and China.
The Black Death, not a virus, caused by the bacterium Yersinia pestis.