O- is a universal blood type. In an emergency if a patient required an immediate blood transfusion and their blood type isn't known, O- will be compatible without the risk of the patient rejecting it. The drawback with O- blood types, is that they can only receive O- blood themselves. It's the most sought after blood in the world and often is in short supply. Some patients with O- blood type even donate their own blood in some cases for their own medical procedures.
Some blood types make the person predisposed to particular diseases or ailments. Heart failure, strokes, cancers etc. Knowing your blood type can set you on the right path to a healthy way of life through an appropriate diet and a good exercise regime.
The reason why I posted this thread, and I've mentioned this before elsewhere, is that O+ blood type is often used in surgery with people that have compromised immune system. Blood type has the ability to give the individual some immunity to many bacterial and viral pathogens.
I'll post a few good-read articles...