Torture is often difficult to prove, particularly when some time has passed between the event and a medical examination, or when the torturers are immune from prosecution. Many torturers around the world use methods designed to have a maximum psychological impact while leaving only minimal physical traces. Medical and Human Rights Organizations worldwide have collaborated to produce the Istanbul Protocol, a document designed to outline common torture methods, consequences of torture, and medico-legal examination techniques. Typically deaths due to torture are shown in an autopsy as being due to "natural causes" like heart attack, inflammation, or embolism due to extreme stress.