I had a few chickens, less than a 1/4 of my flock, that got sick a couple years ago, after I (stupidly) added some new birds. I did injectable Tylan on the sick ones. They recovered and have been healthy. I know there are some folks on this forum that cull, cull, cull at the presentation of illness. However, our birds are my kid's pets. And, the more I learn and speak to vets & poultry professionals, the less alarmed I am about these stories. Chickens can & do get sick and illness can be addresssed with antibiotics and good care.
That said, It is most important to PREVENT ILLNESS THROUGH GOOD BIOSECURITY. So, learn from your experiences and don't repeat your mistakes.