市场抛盘是怎样发生的？

    以一只小鸟作为公司标识的社交传媒公司Twitter，就像是矿井下的金丝雀，最早中招。

    2月初，Twitter股价在一个交易日内大跌24%，从64美元跌至50美元。触发因素是，广告销售增速不及预期。这并没有影响到其他市场投资者。当日，道琼斯工业平均指数上涨188点，创下新高。

    1个多月后，科技股犹如自由落体，直线下跌，似乎每一个坏消息传闻都会加快这些股票的下跌速度。曾在1月底和2月份大涨30%的Facebook，在3月份和4月初下跌了20%。Netflix的股价跌了100美元。

    而且，不只是科技股在下跌。电动汽车公司特斯拉（Tesla）的股价也在下跌。许多生物科技公司的股票亦是如此。

    到底出现了什么状况？如果有什么变化的话，那就是美国经济似乎已有所改善。

    4月10日，科技股权重较高的纳斯达克综合指数(Nasdaq Composite)下跌了129点，创下了超过两年半来最大单日下跌点数，并导致该指数多月来首次跌破4,000点。很多人表示，这只是更大规模抛盘的开始。

    但更大规模的抛盘并未出现——抛盘消失之快，就像来时一样迅猛。虽然Twitter股价尚未完全恢复，但科技股自那以来大多呈现上涨。纳斯达克也已显著高于4月大跌前的水平，仅略低于7月3日创下的14年高点。

    Twitter, the social media company that uses a bird for its corporate logo, was the canary in the coal mine.

    In early February, on a single day of trading, Twitter’s stock fell 24%, to $50 from $64. The impetus, such as it was, was that ad sales were not increasing as fast as expected. The rest of the market shrugged it off. The same day the Dow Jones industrial average rose 188 to a new high.

    A little more than a month later, technology stocks were in free fall, tumbling faster with every whiff of bad news. Facebook shares, which had zoomed up 30% in late January and February, fell 20% in March and early April. Netflix’s shares lost $100.

    And it wasn’t just technology stocks that were tanking. Electric car company Tesla’s shares fell as well. As did many biotechs.

    What had changed? If anything, the economy seemed to have improved.

    On April 10, the technology heavy Nasdaq Composite index lost 129 points, it’s worst single day drop in more than two and a half years—a swoon that sent the index below 4,000 for the first time in months. Many were saying this was just the beginning of a much larger selloff.

    And then—just as quickly as it was—it wasn’t. Though Twitter shares haven’t fully recovered, technology stocks have been mostly rising ever since. Nasdaq is well above where it was before its April dive and is now just a tad off the nearly 14-year high it set on July 3.

    情况又有什么改变吗？求拍醒。

    我从1996年开始做市场报道。这期间，我报道过两次重大的市场恐慌（互联网泡沫破裂和上次金融危机），以及几十次较小的市场恐慌。所有这些来时都毫无征兆，除非我们回过头来看。大多数都没有被察觉，直到造成相当大的损伤。而且，它们会在你最意想不到的时候消失殆尽。

    投资者的心境变化通常都在须臾之间，对未来的看法改变可以像流行感冒一样迅速传播。这是如何发生的？为什么？市场人气波动与病原体、时尚潮流和流言蜚语的传播方式是否存在相似之处？

    在《财富》杂志最新的传染效应专题系列文章中，我和同事们将探索这一谜团般的过程，调查研究诸多事物的传播方式，比如中东呼吸综合征冠状病毒（MERS-coV）病毒、并购传言、图书销售甚至如“自拍”等社会现象。

    What changed, again? Beats me.

    I have been reporting on the markets since 1996. In that time, I have covered two major market panics—the dot.com bust and the financial crisis—and dozens of minor ones. All of them have come without much warning, except in retrospect. Most go unnoticed until a good deal of the damage has already been done. And they disappear when you least expected it.

    The changes in investor mood often happen in a flash—an infection of outlook that can seem as swift as an epidemic of flu. How and why does it happen? And are there similarities between such market mood swings and the way other things—pathogens, fashion trends, gossip—spread?

    In a new Fortune series on Contagion, my colleagues and I set out to explore this murky process, investigating the spread of things as varied as the MERS-coV virus (here and here),M&A rumors, book sales and even a social phenomenon, ahem, such as “the selfie.”