It sounds like SDA virus to me
I'm so sorry.
I have been through this and it's quite hard if you're not used to having a lot of sicklies.... and the fact so many are lost is just horrific.
Put ALL of your ratties onto Baytril and see if a steroid injection is possible for them. I went onto this combinations... Frusemide, Baytril, Bisolven Powder, Prednisalone tablets - after one steroid injection to boost them up. All of the ratties NOT showing symptoms must be on Baytril and if they sound a bit blocked up, probably Bisolven powder too.
Really am going to be thinking about you