In response to your second question, I do not know of any positive purpose the influenza virus serves on an individual person basis. Historically, the 1918 influenza pandemic may have helped bring WWI to a close, at the cost of many lives. The flu hit the front lines across Europe in the first wave of infection. Influenza weakened and cut the numbers of fighting forces, leading to a need for more diplomatic tactics. I am not sure this is exactly a positive outcome because millions of people died from complications related to the flu, but it did help end a global war.