Several years ago we had 2 cases of airborn botulism.  It took the doctors a
while to figure out where it came from because both babies were exclusively
breastfed. The odd thing was that they lived a couple of blocks from each
other (about a quarter of a mile) Apparently a case showed up at another
hospital in our area who also lived near our two cases. That child
unfortunately, did not do well because he was bottle fed.
The health department was called in to find the source. They had the answer
in less than a day. They visited all three homes. The common denominator was
either the family of the baby was putting in an inground pool or the house
next door was putting in a pool. Soil samples were teaming with botulism
spores.  The theory was that the digging produced dust containing the spores
that spread to other homes. The children either inhaled enough spores for a
loading dose or perhaps ingested spores off  dirty fingers etc.
So it doesn't have to be an earthquake to stir up the spores.