There have been several cases in which two children have been chased by a dog, and one child fell while the other kept running. The dog attacked the one who kept running. Of course, not all dogs are so aggressive. The problem for runners is that it is impossible to tell whether a dog has been trained properly. And even if it has, it still may bite.
Dog bites are dangerous for a number of reasons.
If the dog has rabies, you'll need a series of rabies prevention shots. And if you are knocked down, you can break bones, pull muscles or incur some other injury. This is not a dog to mess with. If you do anything confrontational, like throw something or yell or try to chase the dog, you're likely to make the situation worse. The dog may or may not back that up with an attack.