This recent news story brought the topic of pit bull dogs back into the public eye: https://www.usatoday.com/story/news/nation-now/2017/05/02/couple-mauled-son-pit-bull-released-from-crate/309086001/
A few breeds of dogs are especially powerful - capable of killing an adult human. Of those breeds, pit bulls stand out. The others were bred for herding - shepherds and Rottweilers, for instance - their instincts are defensive. They tend to protect their owners and their property, and they can be trained to do limited damage to others. Pit bulls, in contrast, were bred for a sick "sport" - to fight other dogs to the death. They are entirely offensive in their instincts, and these dogs often do not release their grasp on their preys' necks until the dog is unconscious. Their brains are small, inefficient processing units. There is little to train, so it is impossible to be confident that they will not attack at random if their instinct is triggered.
Dogs of all breeds sometimes bite people, but while other dogs may send the injured person to the emergency room, pit bull attacks tend to result in a hospital stay - if the victim survives at all. Also, pit bulls more often attack family members, and their attacks are often completely unprovoked. Not all statistics show pit bulls to be more dangerous than other dog breeds. I believe this is primarily because the owners tend to be so defensive that the breed of the dog is often misreported, provocation is not documented, and most of the articles on the topic are written by pit bull advocates. My mother was attacked by a pit bull as she was walking in the park a few years ago. The owner insisted to the police that her other, smaller dog, was the one that bit my mother, despite clear evidence to the contrary. The police did not care to do a thorough investigation, so the breed of dog on the report was false.
These links to lay summaries include a ResearchGate link so that you can download the actual studies of dog bites from peer-reviewed journals, with statistics gathered from hospitals and emergency rooms, rather than potentially biased newspaper article statistics: http://www.dogsbite.org/dog-bite-statistics-study-characteristics-1616-dog-bite-injuries-level-1-trauma-2016.php and
http://www.dogsbite.org/dog-bite-statistics-study-mortality-mauling-maiming-vicious-dogs-level-1-trauma-2011.php
As a pediatric nurse at a major trauma center, I cared for several victims of severe dog bites. The dog breed was always the same: pit bull. In one case, the father had to shoot the dog to get it to release its jaws from the child. As the child lay unresponsive in the hospital bed, I overheard the father’s parents chastising him: for killing the dog!
When I researched this topic, one physician recommended that owners of pit bulls should adhere to the same regulations as leopard owners. In my opinion owning a pit bill is like routinely handing your toddler a loaded gun. It is possible that nothing bad will ever come of it, but why on earth would you take the risk? What is the upside? Please just choose a more intelligent, predictable breed of dog!