????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.
????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.”