Respiratory illnesses are the
most common illness in backyard flocks. Studies have shown that world wide
80% of chicken flocks get them. These illness are often asymptomatic meaning that your chickens live with them with no signs. They are chronic meaning that your birds are carriers for life and will never get rid of the illness. They typically only show signs when they are stressed from being moved to a new pen, having a change in diet, a change in the weather, etc. Sneezing is a sign that shows in mild cases. Moderate cases may show runny noses or foamy eyes. In severe cases they can get swollen eyes and if things get bad enough the bird can even die. Typically this is something that your flock will survive and isn't severe enough to motivate very many non-commercial flock owners to work for its eradication. The commercial flocks however work very hard to keep it out of their flocks because it leads to significant losses in profit. The longer I keep chickens the less that I am willing to live with in my flock. If a sneezing bird shows any sign of weakness it is culled. That seems cruel when you only have one cockerel to work with, but when you hatch 50 cockerels a year and start culling at 4 months with the goal of finding 3 to keep there is no temptation to keep a weak bird.