When I got laryngitis the morning of a gig out of town, I contacted callers I knew in the town and asked if they were going to the dance if they would call a couple of dances. We had a coordinated "open" mic which worked well and I shared the fee.
When I was a booker and someone canceled, I really appreciated that they gave me a small list of callers they had already contacted, who were available if needed. That made my job so much easier.
The more notice the better of course, but we don't always have that luxury and we need to take care of ourselves and those around us and not spread germs.
Donna Hunt