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 a

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