There is a vaccine you can use for ILT where they do NOT become carriers. We consulted with Peter Brown at first state vet supply and he advised us, this was several years ago. The vaccine was beneficial in saving most of our flock, was an eyedrop that was easy to use, and the surviving birds were (and are) not carriers. (and btw I am not trying to advertise for him here but he is the only one I know that knows the type of vaccine to use, and carries it) We have incorporated several new birds into our flock since then, as well as several hatches of chicks, and the flock has remained healthy with no recurrence. However, I agree that CRD and ILT, among others, is extremely contagious and it is important to take measures to isolate the sick birds. In addition, you should do everything possible to decontaminate everything you can. For large areas, Oxine works best. It is extremely powerful and a little goes a long way. You can mix it with water and use it as a spray, you do not need to follow the instructions on the bottle for commercial decontamination. With Oxine you can even spray the birds themselves with the oxine/ water mixture, and put a few drops in their drinking water to decontaminate the water, too. If it is ILT you will know b/c it is fatal, the birds die quickly. If your hen dies you should get a necropsy within twenty four hours of death if you can, so that you will know what you're dealing with. CRD is not usually fatal but it can be chronic. ILT is not a mandatory cull here, but it is in some states. When our flock had it we were under quarantine by the State, but that turned out to be very useful b/c the State vets were a wealth of information for us, and very kind and cooperative, too.
That being said, remember that your hen may also have something that is curable. Sometimes these cases are isolated and clear up without incident. The most important thing is to treat quickly, don't waste any time. And disinfect as much as possible. Make sure she eats, whatever she likes. If you have her on antibiotics you know you can't eat her eggs for at least three weeks after treatment is done.