yeah im not adding for a while im just looking aroundDon't be adding new birds until you know what killed your current ones. Cold stress can manifest symptoms of some other underlying condition. I read that up to 98% of backyard flocks have MS/MG carriers but may never exhibit CRD symptoms until a time of stress like brooding, molting, prolific egg laying, heatwaves, cold temps, etc, and then you can walk out one morning and find a seemingly healthy bird dead for no apparent reason. Plus AI has been found in Calif. Foster Farms had to destroy 4,000 turkeys in a remote farm and AI can kill a chicken quickly in 24-48 hours after contracting the virus. A necropsy is the only way to know for certain what got your birds. You don't want to add healthy birds until you know what's up for certain. Yes, it's a hassle but that's the responsibility we take on with chicken keeping.
Re DE: I had Leghorns that were particularly sensitive to dusts and always sneezed for an hour after just a plain old dirt/dust bath so I never used powdered treatments on my birds -- I use organic Manna Pro Poultry Protector spray which has no massive danger warnings like the labels of pymethrin or DE dusts and my coop/chickens have been lice/mite free for 31/2 yrs. Poultry Protector can be used as often as you need without danger - just follow label instructions. The following website presented DE pros/cons intelligently IMO: http://www.the-chicken-chick.com/2012/05/diatomaceous-earth-de-benefitrisk.html
Make your own decisions -- I'm just throwing out things for your perusal.