Do air purifiers help with infections or only with pollution?

Both, but with caveats. A HEPA purifier removes airborne particles including those carrying respiratory viruses, so it does reduce infection risk in a closed room — this is best supported for rooms where windows genuinely cannot open. The limits: it must be sized for the room, meaning enough air changes per hour, and it does nothing about close face-to-face contact, which is where most transmission happens. For pollution it is more clearly effective. In most Indian homes, opening windows on two sides does more for infection risk than a purifier, and costs nothing.