There are a variety of settings in which this abnormal clotting process can occur. These include individuals on bed rest (such as during or after a surgical procedure or medical illness, such as heart attack or stroke) or those who are confined and unable to walk for prolonged periods of time (such as during prolonged air or car travel). It can occur in certain families where there is a history of parents or siblings who have suffered from prior blood clots. It can also occur in individuals with active cancer or those undergoing cancer treatment which may predispose the blood to clotting.