Footrot is a common, highly contagious disease of sheep and goats, caused by the interaction of two bacteria: Bacteroides nodosus and Fusobacterium necophorum. B. nodosus is the contagious element of footrot and is only found in sheep/goats that are infected with footrot or in the environment for only a short period of time (2 weeks). F. necrophorum is a normal inhabitant of soil and manure and can always be found where sheep and/or goats are raised.
Foot scald (interdigital dematitis) is an infection of only F. necrophorum and is not contagious. For more information, be sure to read this 29-page publication, Lameness in Sheep, from Great Britain's Department of Environment, Food, and Rural Affairs (DEFRA).