I have some chicks that I purchased from a lady while a naive beginner. These chicks are now almost a year old and I have lost 2 of 5. The first was a month old and had no symptoms. The second became very lethargic and would vomit clear mucus if not held upright. At first I treated with yogurt and acv in water for sour crop. He pooped several times although the seeds seemed whole still and his strength was regained so I released him back into the flock. He became sick a second time and this time I treated him with terra-vet as he seemed much worse (I could almost push him over). After 7 days of small ups, he expired. He was 8 mo. Now I experienced a hen that spit up the same with no other symptoms. What is this? I have a suspicion that they were not vaccinated. If I were to vaccinate now would that make any improvements or is it too late and would be ineffectual?