Before the concept "virus" was misconceived, it was believed that the flu and pneumonia were caused by solar and celestial activity. The name "influenza" is shortened from "Influenza del la stella" or "Influence of the stars". A correlation between number and size of sunspots on the sun and the strength of flu seasons was what led them to give flu this name (in the 15th or 16th century I guess).

In times of fewer sunspots, there are weaker, less deadly flu seasons, and in times of above average solar activity there are more deadly flu seasons. Increased sunspots and activity such as coronal mass ejections from the sun means more cosmic rays and solar radiation penetrates our atmosphere, and there is also increased geomagnetic storm activity.