If it's OPP you have to decide if it's worth doing anything. A LOT of sheep will test positive for OPP. Some of these sheep show symptoms by appearing to have constant low grade pneumonia. They may milk poorly and be on the thin side. The disease is spread by lambs eating milk from mom AND nose to nose contact. Sheep kept in a barn during the winter are much more likely to spread the disease from adult to adult. At the same time, if you test the sheep in your flock, a lot of flocks find the sheep that test positive are their most productive sheep.
Eradication of OPP is usually a several year process. Complications include that there is a difference in accuracy of labs in reading the test results. There are two types of tests you can run. One tends to have some false positives the other has false negatives. If I remember right, ewes lactating tend to have inaccurate test results. Thus, to eradiate your flock of OPP, you have to test all sheep in your flock multiple times. Once, your entire flock is negative for two consecutive years you are considered a OPP negative flock. When sheep test positive in this process you have to decide to either cull the positive sheep or create two separate flocks that have no contact. As stated above you can keep lambs out of the ewes in the positive flock but you can't ever let them eat from a positive ewe. In addition, goats carry a version of this disease and if you use goat colostrum in your flock you can infect your flock that way. You also have to test all new animals coming to your flock at least twice to confirm they are negative before they mix with existing sheep.
In short, having an OPP negative flock is a lot of work, and lot of cost and not very practical if you show or share rams with other breeders. BUT, some flocks are economically devastated by this disease if there ewes are symptomatic.