I free range mine constantly, have come to accept the possibility of predation and potential disease. Put me in
@Sally PB 's camp, with one addendum/clarification.
Fowl Pox, while generally not fatal, moves very very slowly thru a flock, and persists in the environment even after the last chicken has thrown the disease - the virus can remain viable in the scabs falling off into the run, bedding, grass, pasture whatever for some period of time. So, in essence, if you've had it once, its on your property forever.
That's not to say don't free range, however. FP can also be carried by mosquitos from surrounding flocks several hundred feet, so whether your birds are inside the run, or outside free ranging likely won't make any difference to that disease vector. It can also be carried by numerous wild bird species. We can't get it, and there are numerous bird species that have their own varieties which can't be transmitted to chickens, and vice versa.
Essentially, there are no guarantees, only degrees of risk. Perfect safety is a dangerous lie, it means only that there is some risk(s) you are perfectly ignorant of. You have to make a personal assessment of the risk you are willing to tolerate, in the conditions available to you, then re-evaluate as new information becomes available.
I choose to free range, after evaluating my risks, and my tolerance for them.