Please allow me to clarify this statement made by my friend.
Avian viruses often come into a flock, infect the flock, and as long as the birds are young and healthy, often will cause no symptoms. In spite of that, once a flock is exposed to these avian viruses, which can come into your flock on the bottoms of your shoes or by wild birds shedding dander upwind of wind currents your flock happens to be downwind to, to name just a few of the means of transmission, your chickens will be carriers for life and there is no way to eliminate most of these viruses short of very drastic measures.
This is why quarantine is of dubious value. You may have just bought a very healthy chicken that may have been exposed to one of these viruses, and is now a carrier yet is asymptomatic. For this reason, importing adult birds into your flock, except from the most reputable sources, is very risky.
It is true that most of the time, healthy birds, even though exposed to viruses and being carriers, can live out their lives symptom-free, never being sick a day in their lives because their immune systems have made them resistant. Yet they can still pass the virus on to other birds.
Once a chicken shows symptoms, you can treat those symptoms (secondary infections) with an antibiotic. However, all this does is treat those symptoms. It doesn't remove the virus from the bird or the flock. There is no cure for these avian viruses, though vaccination is available for Marek's, and as far as I know, that's the only one.
The best way to find out of your flock carries any of these avian viruses is to have a necropsy done on any bird that has died of mysterious causes. Another way is to have the blood tested from birds from your flock for DNA markers for these viruses, but that obviously is beyond the means of most of our budgets.
Egghead is right in urging you not to panic. There's nothing to be gained from worry and being upset. Most flocks carry these avian viruses and their humans are never even aware of it because the chickens never become sick.
What I suggest is you keep in touch with this friend who has just returned your rooster. If their chicken happens to die, either urge them to get a necropsy or pay for one yourself. That way you both will know for certain if a virus was responsible and which one it is.