The PCV (packed cell volume) of a dog should be in the range of 37-55%. It's a measurement of the percentage of the blood volume that is taken up by red blood cells. 0% is incompatible with life (no red cells to carry oxygen) although I have seen cats with a PCV of 4%. Generally dogs that low would be comatose.
AIHA is a condition where the body's immune system starts attacking the red blood cells and breaking them down. The treatment is a whack of steroids to inhibit the immune response. We had a dog in the vet clinic for two weeks with the condition - he is currently on 50 steroid tablets daily and his PCV is slowly climbing back to normal.
If a dog's PCV is very low, they need a tranfusion just to keep them alive.
Usually an attack of AIHA resolves itself. The steroids are used until the PCV returns to normal and stays there without treatment. Long term prognosis is good althought there may be recurrences. English springer spaniels and cocker spaniels tend to be predisposed to it, i.e. they are more likely to get it than other breeds.