dark clouds
distant mountains
black rain black rain black rain black rain black rain black rain

The Story begins: In Nairobi, Kenya, in the overcast month of May of your year 2XXX, the clouds grew darker than ever before. Spores of a new lichen had co-opted the airborne water droplets. On the 17th, a fuzzy black rain fell. Some hid from the accumulating glop. Others played in it. It made no difference. In a few days they were all very sick. Many died quickly. None got better.
On social media, videos from a football stadium awash in tens of thousands of gasping wheezing people caring for each other sucked in ever more viewers. One obviously shaken celebrity commentator wrote that the sick "as they struggled to breathe, sounded like frightened kittens trying to cough up bagpipes."
Weather spread the lichen.