Blood clot, thrombosis
The willingness of blood to clot is affected by many inputs. One is the number of platelets or thrombocytes. There are other factors in the blood that make clotting more or less likely. The fifth one is called factor five Leiden. Three to five percent of the population at large have a defect in this gene, and that makes their blood less likely to clot (all other factors being equal). These people are at risk of bleeding to death when undergoing major surgery, or when giving birth, etc.
Blood clotting influences: