You don't have to go pandemic or even a contagious disease. Maybe someone tripped and sprained an ankle, wrist, arm, etc. Maybe they fell off a horse and hit their head. Maybe a case of food poisoning means Mr. Collins, Mrs. B, Kitty, and Lydia don't attend the Netherfield ball. Maybe Miss Bingley gets stung by a bee and has an allergic reaction. Mr. Collins tumbles through poison ivy.

So many non-pandemic options.