From Wiki as background:
The Running of the Bulls (in Spanish encierro, from the verb encerrar,to fence in, to lock/shut up, to pen) is a practice that involves running in front of a small group (typically a dozen) of bulls that have been let loose, on a course of a sectioned-off subset of a town’s streets. The most famous running of the bulls is that of the seven-day festival of Sanfermines in honour of San Fermín in Pamplona, although they are held in towns and villages across Spain, Portugal, and in some cities in Mexico, Mesquite, Nevada, and southern France, during the summer. Unlike bullfights, which are performed by professionals, anyone older than 18 may participate in an encierro.
The purpose of this event was in origin to transport the bulls from the off-site corrals where they had spent the night, to the bullring where they would be killed in the evening. Youngsters would jump among them to show off their bravado.
Spanish tradition says the true origin began in North-eastern Spain during the early 14th century. While transporting cattle in order to sell at the market, men would attempt to speed the process by hurrying their cattle using tactics of fear and excitement. After years of this practice, the transportation and hurrying began to turn into a competition, as young adults would attempt to race in front of the bulls and make it safely to their pens without being overtaken. When the popularity of this practice increased and was noticed more and more by the expanding population of Spanish cities, a tradition was created and stands to this day. 
Madrid (CNN) — A 73-year-old Spanish man was gored in the right leg on the first day of the annual running of the bulls in Pamplona and a 21-year-old Japanese runner was dragged by a bull, authorities said.
Those were two of six runners who were rushed to the hospital after the frenetic race down the narrow cobblestone streets of the old Spanish town. One of the others taken to the hospital was a 26-year-old Australian who suffered a knee injury. The Red Cross also treated 72 others for minor injuries at the scene, authorities said.
The runner from Ikeda, Japan — whose gender was not immediately released — was dragged by what appeared to be a bull’s horn, hooked through clothing, on the pavement downhill toward the bullring, images from Spanish state television showed. The runner suffered back injuries.
There have been 15 deaths since record-keeping began in 1924 and thousands of injuries in the tradition that dates back 400 years.
High anticipation for Pamplona bull run
Author Ernest Hemingway wrote about it in his 1920s novel, “The Sun Also Rises,” also published under the title, “Fiesta,” and crowds have poured in from around the globe ever since. The town has since erected a statue to Hemingway outside the bullring.
The running starts at 8 a.m. (2 a.m. ET) for eight consecutive days, through July 14, and lasts just a few minutes, from the corrals through the hard pavement of old town streets to the bullring, where the bulls face certain death against matadors later in the day.