Sheep may contract Sheep Pox from

Being underfed/starving (evidence requested)
Slaughter of nearby sheep with Pox (confirmed)
Randomly (2 adjacent pens, but only 1 has pox - highly unlikely 2nd was just outside radius of nearby slaughter)

In T9 Beta a pen contracted pox with ~50 onions still inside. Another pen 240 coords away had < 10 onions but was safe. Owner of the safe pen took out all but 1 sheep. I killed 1 poxed sheep per minute for 4 minutes and the 240 coord away pen did not contract pox. (Y1-Akhet I-14) - tehm w/ Ashen

Treatment

Once Advanced Animal Husbandry has been researched, give the sheep Sheep Supplement