It can cause sterility, abortion and is not really characterized by symptoms in males so they can pass it all over and no one is aware they are positive without the test. Females can have symptoms if they are pregnant and aborting litters early on or suddenly unable to conceive.
It is not curable and while altering will help some of the issue, once you are known to be positive, the state gets involved. They will more often that not require you to euthanize all positive tested animals. Once one is positive, the whole house must be tested and in some cases the humans too. If anyone comes in contact with infected blood, bodily fluids, such as from birthing then humans can also contract it.
Licking, attempted breeding (which neutered males can still do) will cause a dog to get it.
Not seen often, but very awful if it does happen. I do know of someone who was doing rescue and taking in boarders....those dogs and all her own tested positive in the end all stemming from keeping someones dogs for them...sad situation and one i would never want to face.