I have a question

I was looking at the parish Advent schedule of our old parish, St. Matthew's Cathedral and saw that December 1 is the World AIDS day Mass. The AIDS Mass has been the starting event of Advent for over a decade and nobody seems to know why. And why do we have an AIDS Mass at all? Why not a Diarrhea Day Mass? I'm not being sarcastic. About a million people die from diarrhea every year. Better yet, why don't we have a Malaria Day Mass? Millions of little African children and adults have died after the West banned DDT, in response to Rachel Carson's shaky grasp of science. And what about diabetes and cancer? Are these diseases not horrible? Is the suffering involved somehow unimportant? Why is AIDS alone worthy of a special Mass?