The Dow Jones Industrial Average ended Friday down more than 600 points as a spreading viral outbreak fanned fears about global economic growth, igniting volatility across markets. Investors have quickly soured on the market in recent days, fearful that the outbreak would have far-reaching implications on the global economy. Unlike corporate earnings or economic data, the long-run impact of the virus is trickier to measure for many investors and analysts, injecting even greater uncertainty into markets. …