“Bottom line, it’s going to get worse,” Anthony Fauci, the long-standing director of the National Institute of Allergy and Infectious Diseases, testified.

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How much worse is the pressing question for a nervous American public and its fractious leadership. What happens in the weeks to come is likely to depend on how aggressively the United States acts to contain or slow the novel coronavirus, even at the cost of enormous social and economic disruptions. But what happens also will depend on the virus itself, a new pathogen that is still revealing its true nature.

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Experts have produced forecasts of likely numbers of infections and serious illnesses as well as death tolls, on the basis of what is known about the novel coronavirus and how past epidemics have played out. They suggest that the United States — which has surpassed 1,000 confirmed novel-coronavirus infections and 30 deaths — must prepare for a potentially historic pandemic.

Most coronavirus patients have mild or moderate illnesses and recover without need for hospitalization. The people at elevated risk of serious illness are “older adults,” in the phrasing of the Centers for Disease Control and Prevention, and anyone with an underlying chronic illness such as heart disease, lung disease or diabetes. Data from China showed that the risk increased for people over age 60 and was greatest for people over 80 who had underlying chronic illnesses.

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How this plays out in the United States may not match what happened in China, where the initial outbreak in late December and early January quickly overloaded the health system and led to a high fatality rate.

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Some of the projections for covid-19’s spread in the United States have been grave. A forecast produced last month by Professor James Lawler of the University of Nebraska Medical Center on behalf of the American Hospital Association, for example, put the potential death toll at hundreds of thousands if efforts to mitigate the epidemic fail.

Another forecast, developed by former CDC director Tom Frieden at the nonprofit organization Resolve to Save Lives, found that deaths in the United States could range widely, depending on what percentage of the population becomes infected and how lethal the disease proves to be. Frieden, who oversaw the U.S. response to the 2009 H1N1 influenza pandemic, the 2014 Ebola epidemic and the 2016 Zika epidemic, says that in a worst-case scenario, but one that is not implausible, half the U.S. population would become infected and more than 1 million people would die.

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His team put together a simple table that looks at various scenarios using case fatality ratios ranging from .1, similar to seasonal flu, to .5, a moderately severe pandemic, and 1.0, a severe one. The infection rate ranged from 0.1 percent of the population to 50 percent. That put the range of deaths at 327 (best case) to 1,635,000 (worst case). The deaths would not necessarily happen over a month or a year, but could occur over two or three years, he said.

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U.S. officials have said they are working with 50 academic modeling groups around the country. But so far, no projections of the outbreak’s trajectory have been publicly released by the CDC or the White House coronavirus task force. Still, Frieden said, “anyone who says that they know where this is going with confidence doesn’t know enough about it.”

Communities should undertake social-distancing measures and assume the virus is present, unless proved otherwise, he said. The worst-case scenario can be prevented through common-sense measures, he said, such as holding sporting events in front of empty seats. “Stopping fans from coming into Madison Square Garden is a no-brainer,” he said, alluding to the Big East men’s college basketball tournament scheduled to begin Wednesday evening and run through Saturday.

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In interviews in recent weeks, epidemiologists have suggested that the coronavirus outbreak could be on the same general scale as the 1957 influenza pandemic that many older Americans remember well. They do not think it would be as catastrophic as the 1918 influenza pandemic, which killed an estimated 675,000 Americans and 50 million people worldwide in an era when the population was much smaller.

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The case fatality rate for covid-19 — higher than 3 percent so far globally, according to the WHO — is an evolving number that depends on multiple factors, including access to robust health care. The actual lethality of the virus is hard to determine because many people have mild or no symptoms and are never formally diagnosed as having the virus. Some covid-19 deaths may also have been missed, possibly mistaken as flu-related.

In his Capitol Hill testimony, Fauci said that a plausible covid-19 fatality rate could be 1 percent. That, he said, is 10 times as lethal as seasonal influenza. He was clear: This is more dangerous than the flu.

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“We must be much more serious as a country about what we might expect,” Fauci said. Even places that have little or no known community transmission at the moment need to take action to try to limit infections, he said: “A couple of cases today are going to be many, many cases tomorrow.”

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Knowing how fast and wide the disease can and will spread requires information that the United States lacks, largely because the CDC struggled to produce and distribute reliable coronavirus test kits.

“The problem with forecasting is you have to know where you are before you know where you’re going, and because of the problems with testing, we’re only starting to know where we are,” said Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security. “That’s why there’s not that much forecasting available yet.”

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WHO officials noted on Wednesday that several countries, including South Korea and China, have demonstrated that the viral outbreak can be suppressed and controlled through old-school public health measures of aggressive contact tracing, quarantines and isolation of the sick, social distancing and mobilization of the public to sanitize and prevent transmission.

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“Some countries are struggling with a lack of capacity. Some countries are struggling with a lack of resources. Some countries are struggling with a lack of resolve,” said Tedros Adhanom Ghebreyesus, the WHO’s director general.

Michael Ryan, the executive director of the WHO’s health emergencies program, said, “There’s a real chance to blunt the curve, a real chance to bend the curve. . . . Why would you not use the opportunity?”

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Forecasts have been wrong in the past. Early estimates of the fatality rate for H1N1 in 2009 were much higher than the roughly 0.01 to 0.03 percent it turned out to be. With covid-19, the fatality rate is likely to be in the 0.5 to 1.5 percent range outside Wuhan, according to a report this week from Imperial College London.

China has apparently managed to get some control over the epidemic, reporting no deaths outside of Hubei province, where the city of Wuhan is located, on most days in the past week. China’s success came after an intense, society-wide response that included extreme measures of isolation and social distancing.

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The center of this pandemic is now Europe — to the extent that CDC Director Robert Redfield told Congress, “Europe is the new China.”

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Any virus is far too small to be seen, and a single sneeze can produce hundreds of thousands of infectious particles. A virus like the one in question potentially can remain alive for hours on a surface, such as a door handle or an ATM touch screen.

What makes this virus especially tricky to contain is that it apparently can be spread by people who do not know they’re infected and are not yet showing symptoms of covid-19. That wasn’t the case with SARS, a similar coronavirus that began spreading in China in 2002 and was contained the next year. Because SARS patients shed virus only after they became very sick — nearly 1 in 10 died of the disease — health-care systems were able to identify and isolate patients and keep the total number of cases below 10,000 globally. This new virus already has sickened well over 100,000 people, and the number of infections — including people with mild illness or no illness — is much higher.

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One fact might serve as a steadying handrail as people worry about how bad this situation will become: The virus (officially SARS-CoV-2) does not spread as efficiently as the influenza virus.

This coronavirus has an estimated reproduction rate — meaning the number of infections generated by each infected person — somewhere between 2 and 3. For the epidemic to end, that reproduction rate (“R-nought,” in medical lingo) has to fall below 1. The influenza virus’s rate is about 1.5.

But the R-nought isn’t an intrinsic feature of the virus, said Gary Whittaker, an infectious-disease expert at Cornell University. Contagiousness is a slippery metric because so much depends on the social and physical environment in which the virus is spreading. In crowded cities, it spreads more easily than in wide-open areas, and the R-nought can be lowered through a variety of interventions, including, eventually, a vaccine.

More important is the metric known as the serial interval. That refers to the average amount of time it takes for one infection to lead to another infection in the chain of transmission. That’s about four days for the new coronavirus. SARS was about seven days. Influenza is only a couple of days. The flu spreads so quickly that it’s essentially impossible to contain through social distancing. But the slower spread of the coronavirus makes social distancing the right strategy now, Whittaker said.

“The social distancing will bring the R-nought down and ideally will bring it below 1,” he told The Washington Post on Wednesday.