Weren’t HIV and/or Swine Flu engineered by humans?
As the Dutchess said, viruses mutate rapidly, and there have always been many. What’s new are the transportation, the news media, the medical awareness, and the comfortable expectations of people not dying due to epidemics. Just 100 years ago, more people died from disease epidemics after the First World War, than died in the war.
Also, the more humans there are everywhere, the more population density, the more pervasive they are, they more hospitals we have, and the more industrialized slaughterhouses and atrocious industrialized farming practices and other centers for the production and distribution of germs, the more germ research, the more opportunity there is for disease generation.