Hey there, let me give this a shot:
1. Powder CAN harbour bacteria, but not anywhere nearly as easily as cream/liquid products, so though it's unlikely you can 'catch' something, you might want to take extra care with eyeshadows as you wouldn't want to get an eye infection.
2. I spritz powder products with alcohol lightly; if the product looks very used (which is unlikely, in that I wouldn't have it if it were), I might scrape the upper layer off lightly and spritz, just to be extra cautious.
3. You know the medical alcohol swabs they use on your arm/buttock before giving you an injection? You can use these to swipe but don't rub too hard, and make sure it's in one quick swipe, in one direction only so as not to ruin the surface of your powder product.
I've followed all of the above myself, and have yet to experience any sort of infection or irritation on skin etc. You may also want to make sure that the tools you use to apply products are clean, and are going onto a clean face - the natural oils in our face can cause a light build-up on some powder products.
