U.S. stocks staged a fresh plunge Thursday, pushing the S&P 500 index and Nasdaq Composite into a bear market and officially ending a record-setting, 11-year-old bull-market run--and in spectacular fashion. Uncertainties around the economic impact of COVID-19, the infectious disease that originated in Wuhan, China in December and has infected 128,000 people, has decimated investor buying appetite. Worries about accelerating problems in the oil patch, also helped to fuel almost-indiscriminate selling of assets considered risky and haven assets like bonds and gold alike. The S&P 500 index SPX, -1.11% fell 9.51% to 2,480.64, the Dow Jones Industrial Average DJIA, -0.87% tumbled 9.99% to end at 21,200.62, representing the worst percentage drop for blue chips since October of 1987. Meanwhile, the Nasdaq Composite Index COMP, -1.07% gave up 9.43% to close at 7,201.80. On Wednesday, the Dow fell into a bear market and the Nasdaq and S&P 500 joined the benchmark on Thursday.A drop of at least 20% from a recent record peak is the widely accepted definition for a bear market. "While there continues to be human tragedy, the latest developments also add to investor uncertainty. There is little doubt that there will be a dramatic short-term impact on the economy. The probability of at least one quarter of negative economic growth is high," wrote strategists at SunTrust Advisory in a late-Thursday note. "Therefore, a key question for longer-term investors is: Are the economic effects of the coronavirus likely to be similar to a storm or will it cause permanent damage to the economy?" the analysts wrote.