How much does ventilation actually matter for indoor infection risk?

A great deal, and it is the most under-used lever in Indian homes and offices. Respiratory infections spread substantially through small airborne particles that linger, so diluting indoor air reduces the dose people inhale. Opening windows on two sides to create cross-ventilation, even for short periods, clears air far faster than a single window. An air conditioner recirculating the same air does nothing for this, and a ceiling fan mixes air without removing it. Where windows cannot open, a HEPA purifier sized for the room helps; in most Indian climates, opening a window is simpler and free.