HELP!! Chicks appear suddenly with red bums and are dead!

[princess2014]

Our chicks are between 1 and 2 weeks old. All seem very healthy and lively. All are mixed heritage breeds with some purebred heritage breeds. Just yesterday we find one almost dead with a "picked" bottom. I assumed he got a wee injury and so everyone started picking at him. Then this afternoon my husband went in and another one dead with a red bum, not necessarily looking like it was picked. Now tonight, another one. We are with these chicks ALOT during the day and this is happening very suddenly and I have raised chicks for years and never seen this! Please any advice would be great! Picture attached.