Rat URI or something else?

I had two rats out of five who had URI's for about six months, they both died the start of this year. Start of feb oscar started snotty breathing and was put strait on antibiotics then week later had steroid ijection, seems to have helped.. Now recently this week, sally has started snotty breathing, then doris started, now daisy! What on earth is going on? The vets say its just bad luck. I dont understand, i have NO airfresheners, i dont use body spray or perfume, they have vetbed on the bottom of their cages(big wire cages) and shredded paper to nest and paper cat litter in the trays! Whats causing these infections? I clean the cages on sundays and Wednesdays, when i change litter trays and wash vetbed with milton and a tiny amount of washing liquid, rinse then tumble(tumbler takes out any smell of the milton or washing liquid, they smell of nothing when they come out, but are allowed to air out until the next clean) I wipe down shelves daily and change hammocks once a week. I only hoover once a week as i know it can throw up dust.

Whats causing these infecions? They seemed very spread out until this year to be anything contagious? I know all rats have myco, and things cause it to flare but ive tried my best to avoid it!

Anyone help me?