I have had one of my hoggets with orf in early spring, no others got infected, even though they were all at the feeding through together... cream and antibiotic injection (and separated them for a couple of days after I had seen it) sorted it. Even though my flock came onto this land after it had just been seeded up from arable and no new sheep for two years... so it must be in the ground, fence posts...
It's not so serious if it is outside lambing/sucking time I think.