Why, for instance, was there such an enormous death toll in northern Italy, but not the rest of the country? Just three contiguous regions in northern Italy have 25,000 of the country’s nearly 36,000 total deaths; just one region, Lombardy, has about 17,000 deaths. Almost all of these were concentrated in the first few months of the outbreak. What happened in Quito, Ecuador, in April, when so many thousands died so quickly that bodies were abandoned in the sidewalks and streets? Why, in the spring of 2020, did so few cities account for a substantial portion of global deaths, while many others with similar density, weather, age distribution, and travel patterns were spared? What can we really learn from Sweden, hailed as a great success by some because of its low case counts and deaths as the rest of Europe experiences a second wave, and as a big failure by others because it did not lock down and suffered excessive death rates earlier in the pandemic? Why did widespread predictions of catastrophe in Japan not bear out? The baffling examples go on.

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By now many people have heard about R0—the basic reproductive number of a pathogen, a measure of its contagiousness on average. But unless you’ve been reading scientific journals, you’re less likely to have encountered k, the measure of its dispersion. The definition of k is a mouthful, but it’s simply a way of asking whether a virus spreads in a steady manner or in big bursts, whereby one person infects many, all at once. After nine months of collecting epidemiological data, we know that this is an overdispersed pathogen, meaning that it tends to spread in clusters, but this knowledge has not yet fully entered our way of thinking about the pandemic—or our preventive practices.

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There are COVID-19 incidents in which a single person likely infected 80 percent or more of the people in the room in just a few hours. But, at other times, COVID-19 can be surprisingly much less contagious. Overdispersion and super-spreading of this virus are found in research across the globe. A growing number of studies estimate that a majority of infected people may not infect a single other person. A recent paper found that in Hong Kong, which had extensive testing and contact tracing, about 19 percent of cases were responsible for 80 percent of transmission, while 69 percent of cases did not infect another person. This finding is not rare: Multiple studies from the beginning have suggested that as few as 10 to 20 percent of infected people may be responsible for as much as 80 to 90 percent of transmission, and that many people barely transmit it.

This highly skewed, imbalanced distribution means that an early run of bad luck with a few super-spreading events, or clusters, can produce dramatically different outcomes even for otherwise similar countries. Scientists looked globally at known early-introduction events, in which an infected person comes into a country, and found that in some places, such imported cases led to no deaths or known infections, while in others, they sparked sizable outbreaks. Using genomic analysis, researchers in New Zealand looked at more than half the confirmed cases in the country and found a staggering 277 separate introductions in the early months, but also that only 19 percent of introductions led to more than one additional case. A recent review shows that this may even be true in congregate living spaces, such as nursing homes, and that multiple introductions may be necessary before an outbreak takes off. Meanwhile, in Daegu, South Korea, just one woman, dubbed Patient 31, generated more than 5,000 known cases in a megachurch cluster.

The only time Trump shut up last night was when he was asked to denounce white supremacy.

Donald Trump was talking at Joe Biden, and Joe Biden was talking to America.

That’s what happened Tuesday night during the first presidential debate of 2020. Trump was focused on smearing his opponent, and Biden was trying to connect with voters. Throughout the evening, Trump kept his sneering look fixed upon Biden, as he heaped abuse on Biden (and Biden’s son, Hunter). Trump rarely addressed voters. It was as if he was only bent on creating content for Sean Hannity. Biden, in a stark comparison, often peered into the camera and attempted to speak directly with viewers. At one point, Biden, while responding to yet another Trump assault on Hunter Biden, brought the debate straight to those watching: “This is not about my family or his family. It’s about your family.”

It was clear that the two men viewed their tasks rather differently. Trump’s goal was to bully and soil Biden. He constantly interrupted and talked over Biden, insulting him as much as he could (“there’s nothing smart about you, Joe”) and practically turning moderator Chris Wallace, the Fox News host, into road kill. He acted like a pathetic narcissist: Biden is the threat to his existence, destroying Biden is his only priority. He could not imagine a strategy beyond a DEFCON-1 personal attack. Reaching voters who were not already die-hard Trump devotees was not on his to-do list. Trump doesn’t give a damn about that. He only cares about the Fox-watching cultists who are already in his pocket.

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But Biden did all he could to pull out of the black hole created by Trump’s cray-cray. He often refrained from taking the bait. And when he could, he tried to escape the vortex and tell voters what he would do to help them. On health care. The economy. Climate change. Systemic racism. Sure, he got in his licks against Trump, calling him a “clown” and a “racist”—telling Trump to “shush” and “shut up,” as Trump repeatedly violated the debate rules he had agreed to. But Biden saw his primary objective as selling himself as a normal, caring guy who would go to work for you. He was aiming to appeal to voters, especially those not decided. He is an old-fashioned pol who does believe that trope that success in politics is about addition—adding voters to your coalition.