This is precisely why I DON'T buy pullets! My new birds come as day old chicks from NPIP hatcheries ONLY. Chickens are prone to diseases, and then they're carriers. They contaminate the coop, and then new birds become infected. HUGE issue!
You can treat the birds with Tylan, injectable is the fastest acting, but added to water is okay, too. It works for most infections. Yes, its okay to keep these birds if you're willing to take the time, spend the money, put in the effort, take the chance of infecting your other birds, and future birds. I have also used Oxine spray to fog & spray the coops on a regular basis, and to mist any sick or wheezey birds. As we NEVER sell birds, even eggs, I chose to separate the birds in another building. Have not had ongoing issues for years, but will keep these birds in quarantined area till their eventual demise.
Best of luck!