Well, chronic respiratory diseases in chickens are usually brought in from other flocks, or people who don’t mind selling birds who have been exposed to diseases. How many birds do you have? When one has a respiratory disease, it makes the whole flock carriers for life usually. The foamy eye sound like mycoplasma (MG.) I would close the flock to birds going out or coming in. You can chose to treat the sick birds with antibiotics which can get expensive, or to cull sick birds. Most state vets will do a necropsy/autopsy on a sick bird to give you a diagnosis of the disease. That is a good way to know for sure. These birds may give you some eggs, but sometimes they may not lay well. Some chose to cull the flock and start over, but it depends on how big a flock one has. You can wait until the last bird dies and then get new chickens. I don’t buy birds from anyone anymore, but buy healthy baby chicks from a feed store or a hatchery. Good luck, and sorry about the illness.