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at least sorely testing virtually every health system on the planet, a bad flu season could have been disastrous. But there was not a bad flu season. In fact, there was hardly any flu season at all. Between the flu seasons of 2019-20 and 2020-21, cases dropped 99 percent. As I write this, in October 2021, one particular type of flu known as B/Yamagata has not been detected anywhere in the world since March 2020. Other respiratory viruses also dropped by quite a bit. By the time you read this book, of course, many flu strains will be returning to their normal level. Flu strains have a way of disappearing for long periods and then suddenly recurring without explanation. But the huge decline in cases across the board is unmistakable, however long it lasts, and there is only one plausible explanation: Nonpharmaceutical interventions made a dramatic difference in reducing flu transmission when combined with the prior immunity and vaccinations that people had. This is great news, and not only because it means we did not have a disastrous twindemic of flu and COVID in 2020-21. It's also reason to be hopeful that if there's an outbreak of a bad flu in the future, NPIs can help keep it from turning into a pandemic. While it's possible that we'll see an influenza so transmissible that it overwhelms our best efforts to contain it without a vaccine, it is reassuring to have more evidence that NPIs are effective against the common strains we know about today. And we now have solid evidence that, when paired with vaccines, NPIs could help us eventually eradicate every strain of flu, and every strain of coronavirus too. We should use contact tracing to find superspreaders. Depending on which country you live in, if you tested positive for the COVID virus, you may have been visited by someone asking about all the people you might have come into contact