NEW YORK - A $1 billion copyright infringement lawsuit over YouTube's ability to keep copyrighted material off its popular video-sharing site threatens how hundreds of millions of people exchange all kinds of information on the Internet, owner Google Inc. said. The company's lawyers made the claim in papers filed in U.S. District Court in Manhattan as Google responded to Viacom Inc.'s latest lawsuit alleging that the Internet has led to "an explosion of copyright infringement" by YouTube and others. The back-and-forth between the companies has intensified since Viacom brought its lawsuit last year, saying it was owed damages for the...