Id say this is a very fast acting infection. I just bought 2 roosters and a hen and introduced them into the flock without giving them 30 days isolation. I dont really have a place to keep them other than in the pen. But even then...they all seemed healthy and showed no signs of sickness. Up until about 3 days ago when I noticed my chickens sneezing a lot. I would say all of them at this point are sneezing and sitting with their ruffled and neck hiding inside feathers. Still eating but not very active at all. The first couple of days I thought it was due to the weather change. Most nights it only got down to 50 degrees or so but the past 3 or 4 days it got down to the upper 20's. Anyways....theyre sneezing a lot. No snot on their beaks as far as i can see. but their eyes arent staying opened...they shut them often either fully or at least halfway. So to the internet I went and saw it was recommended that I give them Duramycin. Main ingredient is tetracycline...which i have some fish cycline. 250mg packets. I put one packet into 8 cups of water and put it out for the chickens. This should do the trick right? As I approached the pen this morning i noticed my youngest rooster (about 13 weeks) was still in the coop and all the others were already out and eating. His comb was very dark red insead of bright ...i knew something was up with him. So i picked him up (no fight in him) and put him near the water...he wouldnt drink so i got a syringe and force fed him some of the water with antibiotics. He died right in my arms. Not sure if i choked him with water or if he really was that far gone. Pretty upset he was by far the friendliest of my flock. Like i said this seems to be very fast acting...its only been a few days. But I figured I would post on here and see if anyone has any recommendations on anything else i should do to get them back on the mend. Its been a rough couple of months with them and dont think i can take losing anymore. I know not to eat any eggs for at least 21 days after theyre better because of the antibiotics being in their system. But any other tips would be great....Thanks.