Tylan is given for 5 days total. Do some reading about mycoplasma gallisepticum (MG.) We don’t know for sure what your chickens have, but if it responds to Tylan it may well be. MG is a chronic respiratory disease, and you should realize that your flock may be carriers for life. The rooster may have good immunity, and is not getting sick because he has fought off the illness. He won’t infect them if he gets sick, because the disease is already there. They may have some immunity to it in the future, but some may get sick again someday if they are stressed. That could be during a molt or during a very cold winter. Many backyard flocks are positive for MG, since it can be spread by birds. There is one link in post 18 about the common diseases, and here are 2 more about MG:
https://thepoultrysite.com/disease-...tion-m-g-chronic-respiratory-disease-chickens
http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/poultry/FS-1008 Recognizing and Preventing Mycoplasma gallisepticum (MG) Infecti....pdf