Legionella generally comes from fresh water sources. If you're worried about it filter your water.

Airborne contagions do not spread from bringing in outside air and tempering it via an HRV/ERV. The risks rise when you do not bring in fresh air but rather recirculate indoor air, but these can be reduced by using MERV 13+ filters:

* https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7127325/

Use a HEPA filter. But even without a HEPA filter, for most shared indoor spaces (restaurants), it’d better to exchange with the outside air than to recirculate unfiltered inside air.

Plenty of options to fix that problem.

E.g. a cheap addition to existing installations: aircode.se

Limited but sane research from around 2013(?) points to even cheaper options of ventilation treatment.