Exsanguination
First aid saved many lives during the Boston Marathon bombings, when ordinary people used belts for tourniquets or used their own hands to press against bleeding wounds.  The three people who died in the bombings died right away. There were many other people who were hurt badly enough that they could have exsanguinated. For example, some people's legs were blown off by the bombs. However, because of the help of ordinary people giving first aid (and because of emergency medical services), everyone who did not die right away survived.