Clot

Clotting describes the process whereby platelets, a component in the blood, group together to prevent the flow of blood past a point. Clotting is a normal process and stops bleeding when a blood vessel is cut or otherwise damaged, either internally or externally.
However, in some circumstances, a clot can form in an otherwise healthy blood vessel, preventing blood from flowing normally through the blood stream past the point of the clot. Clots can cause strokes, heart attacks, or infarctions in other parts of the body.
There are several conditions that can either prevent blood from clotting, or allow the blood to clot too freely. Hemophilia is the best known of the conditions that can prevent clotting.
Several drugs, known as blood thinners, can prevent clots from forming properly. However, the risk is that the patient could bleed to death from even minor wounds or injuries.
Platelets can be given to patients whose blood is not clotting properly. However, there are several risks, such as exposing the patient to blood-borne infection. Many hemophiliacs were exposed to AIDS and hepatitis when they were treated with platelets.