In my (Sue's) experience, the biggest single factor in health of sheep is the correct balance of minerals, which permits their immune systems to function optimally.  Thus for example, most sheep should not have much copper, though all need some, with downland and Texel sheep being particularly susceptible to copper poisoning, fine-woolled sheep intermediate, while Gotlands and Finnsheep are more tolerant than other breeds and indeed need a greater amount.

This is why it is important never to feed sheep with pig or poultry feed, which contains too much copper.  My sheep broke into the feed store and gorged themselves on pig food once and the greediest had the worst hangovers – luckily a vitamin injection was sufficient to get them back into a state of normality (as far as Gotlands are ever normal …).