Thrombosis is the formation of a blood clot (thrombus) in a blood vessel. It can occur when a blood vessel is injured by infection or trauma. If the injury is caused by bacteria, they may eventually disintegrate the clot, causing infectious particles to be carried through the bloodstream to establish secondary infections (metastases) at other sites. Alternatively, the entire clot can break loose, especially in the absence of infection, and be carried through the circulatory system until it reaches a blood vessel too narrow to pass. There it lodges and instantly blocks circulation, a condition called embolism. Tissues normally supplied by the obstructed vessel disintegrate and die unless their needs can be met by other nearby blood vessels called collaterals. The clinical consequences of an embolism depend on where it occurs. If it occurs in the brain, it can cause a stroke. If it occurs in a coronary artery, it can cause a heart attack.