????以一只小鳥作為公司標識的社交傳媒公司Twitter，就像是礦井下的金絲雀，最早中招。

????2月初，Twitter股價在一個交易日內大跌24%，從64美元跌至50美元。觸發因素是，廣告銷售增速不及預期。這并沒有影響到其他市場投資者。當日，道瓊斯工業平均指數上漲188點，創下新高。

????1個多月后，科技股猶如自由落體，直線下跌，似乎每一個壞消息傳聞都會加快這些股票的下跌速度。曾在1月底和2月份大漲30%的Facebook，在3月份和4月初下跌了20%。Netflix的股價跌了100美元。

????而且，不只是科技股在下跌。電動汽車公司特斯拉（Tesla）的股價也在下跌。許多生物科技公司的股票亦是如此。

????到底出現了什么狀況？如果有什么變化的話，那就是美國經濟似乎已有所改善。

????4月10日，科技股權重較高的納斯達克綜合指數(Nasdaq Composite)下跌了129點，創下了超過兩年半來最大單日下跌點數，并導致該指數多月來首次跌破4,000點。很多人表示，這只是更大規模拋盤的開始。

????但更大規模的拋盤并未出現——拋盤消失之快，就像來時一樣迅猛。雖然Twitter股價尚未完全恢復，但科技股自那以來大多呈現上漲。納斯達克也已顯著高于4月大跌前的水平，僅略低于7月3日創下的14年高點。

????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.

????情況又有什么改變嗎？求拍醒。

????我從1996年開始做市場報道。這期間，我報道過兩次重大的市場恐慌（互聯網泡沫破裂和上次金融危機），以及幾十次較小的市場恐慌。所有這些來時都毫無征兆，除非我們回過頭來看。大多數都沒有被察覺，直到造成相當大的損傷。而且，它們會在你最意想不到的時候消失殆盡。

????投資者的心境變化通常都在須臾之間，對未來的看法改變可以像流行感冒一樣迅速傳播。這是如何發生的？為什么？市場人氣波動與病原體、時尚潮流和流言蜚語的傳播方式是否存在相似之處？

????在《財富》雜志最新的傳染效應專題系列文章中，我和同事們將探索這一謎團般的過程，調查研究諸多事物的傳播方式，比如中東呼吸綜合征冠狀病毒（MERS-coV）病毒、并購傳言、圖書銷售甚至如“自拍”等社會現象。

????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.”