I have 3 oversized air purifiers in my home.

I don't really know if they help to prevent infections but my wife has Long Covid and every time there is too much dust, pollen or smoke around us she gets pretty bad.

So we decided to buy them with little proof after she noticed she feels somewhat better when staying in a room with one. It could even be partially placebo but for the days with too much particles from outside I think it really helps.

Maybe we should rather have a full control of air flows in our home but that would be a huge investment for a very old house, also having air flowing with windows partially open on dry days also seems to help. Humidity ( and certainly mold) also has a clear negative effect so I had to buy a dehumidifier too