Hi, fairly new chick owner here. I have an established flock of 3. In February I decided to expand my flock with some chicks. I got two from a farm, not a hatchery. This is the worst luck with chicks I’ve ever had. First one had a respiratory infection, it was treated, and recovered. ( I now know that she is a carrier for life...I didn’t know that a month ago) Then the other died several weeks later due to sour crop. After the she passed I got two more chicks from a real hatchery, (they were four weeks old when I got them).It’s been a month and now one of the younger chicks has the same infection (runny nose, watery eyes, sneezing) that the other did two months prior.We’ve been treating with probiotics, ACV, electrolytes etc. I’ve eliminated environmental factors as possible causes. Im not sure where they got the infection, if perhaps the first set of chicks had it, and passed it to the new chicks, or if my establish hens had it and I transferred it to the young chicks. However my hens have never been symptomatic of anything. I don’t know if I should cull and start over in a few months, or continue treating until all the chicks are well and they can join my adult hens. I don’t want to cull, but I have to protect my older gals. But if they’re the ones that gave the chicks the infection in the first place, it’s a moot point to start over, as any new birds I bring in will be at risk. I am overwhelmed and need help. Thanks.