????利?德羅根對自己首次聽說以色列互聯網安全公司Radware將被收購時的情形記憶猶新。

????當時，德羅根正管理著一只動力股對沖基金，而且一直在觀察在美國上市的Radware。他回憶道：“我掌握的所有情況都表明我應該持有這只股票，只是當時我還沒有動手。”隨后，德羅根關注的一位頗有聲望的交易員在Twitter上羅列了一大堆自己對這只股票的看法，而且預計Radware的股價將上漲。這位交易員的結論是，Radware一定是別人的收購目標。德羅根說：“就是這句話讓我從觀望變為行動。”隨后，德羅根開始迅速吸納Radware的股票。

????收購傳聞在一周內傳遍了華爾街。分析師都把Radware列為有吸引力的個股。一家以色列報紙還引述未具名消息人士的話說，IBM和惠普（Hewlett-Packard）提出以近10億美元的價格收購Radware，而后者正在考慮此事。美國各家網站和新聞媒體紛紛轉載這篇報道，Radware的股價也因此大漲特漲。德羅根說：“當時這看來是真的。”

????這次收購一直沒有付諸實施。但對德羅根來說，這沒有關系——塵埃落定時他早已抽身離去，而且獲利不菲。他說：“如果你手里的某只股票因為傳聞在一天內上漲了40%，那就要把它賣掉。對唾手而得的收獲不能要求太多?！?/p>

????從這個典型事例中可以看出傳聞對市場的影響力。它也表明，在別人收到風聲前對恰當的傳聞加以利用可能得到怎樣的回報。借此類炒作賺錢的機會往往讓人很難抗拒，就連職業投資者也是如此，盡管他們，包括《財富》（Fortune）雜志撰寫本文時采訪的所有證券投資經理，發誓說自己絕不會把傳聞這種如此不靠譜的東西作為交易依據。畢竟，他們的工作就是關注所有可能影響股票的東西。而和大多數其他因素相比，并購，甚至是并購傳聞，能讓股價出現更大的起伏。

????有時候，并購傳聞猶如瘟疫一般在市場上到處傳播，最終成為一個自我實現的預言，它對股價的影響就好像相關公告剛剛發布一樣。

????在《財富》雜志以傳染為主題的系列文章中，我和同事們將對這個現象進行探究。這個題目既會覆蓋典型（而且致命）的“傳染性冠狀病毒”，也會囊括象征意味更濃的“病毒性癥狀”，比如股市出現拋售潮，大賣特賣的最暢銷書籍，乃至“自拍”的流行。

????并購傳聞的傳播效果很明顯，但對許多市場專家來說，這背后的機制仍然完全是個謎。我們知道，和股票有關的傳聞差不多就是交易員日常生活的一部分。但只有一部分傳聞引起交易員的廣泛關注，其中甚至包括一些看起來根本沒有什么依據的傳聞，為什么會出現這樣的情況還遠遠沒有解釋清楚。

????股票研究和交易機構Summer Street Research Partners醫藥科技股分析師馬克?蘭迪研究了大量并購傳聞。他指出：“傳聞當然會影響市場。有人在市場上投資，有人在市場上投機——老實說，有時候投機的那部分人給投資的人帶來了機會。”蘭迪還提醒我們：“投資領域甚至有句老話，出現傳聞時買入，傳聞兌現時賣出。”

????Leigh Drogen remembers exactly how he first heard that Radware was going to be acquired.

????At the time, Drogen was running a hedge fund of momentum stocks, and he’d been watching Radware, an Israel-based Internet security firm listed in the U.S. “Everything lined up for me that I should be owning this company, but I hadn’t pulled the trigger yet,” he remembers. Then on Twitter, a respected trader Drogen follows tweeted that a slew of options trades on Radware stock—bets that its shares would rise—had just been made. The trader’s conclusion: The company was definitely a takeover target. “That was the thing that put me over the edge,” says Drogen, who quickly loaded up on Radware shares.

????Within a week, Wall Street was abuzz with acquisition rumors: Analysts had labeled Radware an attractive target, and an Israeli newspaper, citing anonymous sources, reported that the company was considering a nearly $1 billion offer from IBM or Hewlett-Packard . The story was picked up by American blogs and news outlets, sending Radware’s stock through the roof. “It seemed real,” Drogen recalls.

????The deal never materialized, but that didn’t matter to Drogen, who was already long gone by the time the dust settled—taking a handsome profit with him. “If one of your stocks goes up 40% in one day on a rumor, you get rid of it,” he says. “You don’t look a gift horse in the mouth.”

????It’s a classic example of the power that gossip wields in the market—and the potential rewards for those who play the right rumor before everybody else catches wind of it. The chance to capitalize on such speculation is hard to resist, even for professional investors who swear that they would never trade on anything so flimsy as a rumor (including every portfolio manager Fortune interviewed for this story). It’s their job, after all, to look out for anything that could impact a stock—and mergers and acquisitions, even rumored ones, can sway stock prices more dramatically than most other events.

????Sometimes, when M&A speculation catches on, it becomes so contagious it spreads to the market itself, becoming a self-fulfilling prophecy—moving prices as though a deal had just been announced.

????In a new Fortune series on Contagion, my colleagues and I set out to explore this phenomenon—from the classically (and fatally) contagious MERS-coV virus (here and here) to more figuratively viral manifestations like stock market selloffs, runaway bestsellers and even the advent of “the selfie.”

????With M&A rumors, the contagion effect is obvious, but the mechanics behind it are a complete mystery to many market experts. We know that stock-related rumors are an almost daily part of the trading life. Much less clear is why certain bits of gossip—including some that seem to have little substance at all—catch a wave of trader attention.

????“Certainly, rumors move the market,” says Mark Landy, a medical technology stock analyst with Summer Street Research Partners, who has vetted his fair share of M&A gossip. “You have people who invest in the market and people who gamble in the market—and honestly sometimes, the gambling portion of the investment community gives the investment part an opportunity.” There’s even an old investing saying, Landy reminds us: “You buy the rumor and you sell the news.”