Hi there, so sorry to hear about your chicks. I am not sure what kind of chicks you have or if this even relates to you..however, I had a horrible experience with our last batch of broiler chicks (all were vaccinated in the shell)..came from a reputable hatchery and started sneezing maybe two days later..then they started dropping off after that. The hatchery recommended Neotetramed, which I put in the waterers immediately. Seemed to help..death rate slowed right down which was a relief considering I was finding at least two (gone) every day. In the end, the death toll was 1/3 of the 60 I had..and left rethinking how to do this better next time..I have had great success with our previous batches which is why it stumped me so badly..even when temps, cleanliness and everything is followed to a tee on my end of things..still there can be something as small as catching a chill during transport from the hatchery that could wipe them out..since I live in a cooler climate this can be a big problem..I am leaning towards a local person who I heard was selling his broiler chicks at 4 wks of age..when they are ready to be off the brooder lamps..hope this helps a little..