A new smartphone game that has fans chasing virtual monsters through real-life city streets has boosted the market value of Nintendo Co. by $9 billion in just a few days, as investors cheered the Japanese videogame maker’s first mobile-gaming hit.

“Pokémon Go,” which was released less than a week ago, already ranks among the most-downloaded and top-grossing smartphone apps. But its sudden success is raising questions about the privacy and security technologies fueling the game, as well as the physical risks of playing it.

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