There are some good-enough-for-now shortcuts based on what some hospitals have been doing to convert regular rooms into negative-pressure ICU rooms. The same strategy can generally be employed anywhere with central air, though:
1) Either fit a building's air filtration with HEPA-standard filters, or else employ air scrubbers (supplementary filtration, not cooling) in poorly-ventilated areas where stale air & aerosols are slow to disburse. Decent air scrubbers go for $700 - $1,500+ depending on capacity (how much air they move per unit of time).
2) Jury-rig in-room systems to bypass a building's recirculated air. Basically, turn a central air system into a bunch of single-room systems (esp. if those single rooms had been stale-air bottlenecks). A window to the outdoors is required to fit a room this way. The quick-&-dirty method: remove a window, cover with plywood and seal edges, cut hole in plywood, seal flexible ductwork to that hole, run ductwork to the room's air intake to ensure that outside air is always getting fed back in to the system as inside air and accumulated aerosols are released via outdoor condenser fans. Yes, this is extra energy overhead for a system meant to handle and cool recirculated indoor air -- it a given building's unit is on its last legs, it will have to be upgraded. And if upgraded, something "overbuilt" is recommended (e.g. a unit meant for 10,000 sq ft might now be more appropriate for a 5,000 sq ft space constantly exchanging air with the outdoors).