NEW YORK (NYTIMES) - Making sense of the coronavirus pandemic requires getting up to speed on semantics as much as epidemiology.

Government officials and health care professionals toss off mentions of mortality rates, flattening the curve and lockdowns, assuming that we know what they mean. But the terms mean different things from country to country, state to state, even city to city and person to person.

Officials use the same phrases about mass testing, caseloads and deaths to describe very different situations. That makes it hard to give clear answers to vital questions: How bad are things? Where are they headed?

People search for insight by comparing their countries to those that are further along in the epidemic. But if the terms are misleading or used in differing ways, the comparisons are flawed. Also, the statistics and vocabulary offer a false sense of precision while in reality, the information we have shows only a fraction of what's going on.

"The new cases or deaths each day are given as exact numbers and we're trained to take that at face value," said Associate Professor Mark N. Lurie, an epidemiologist at Brown University's School of Public Health. "But those are far from exact, they're deeply flawed, and their meaning varies from place to place and from time period to time period."

CONFIRMED CASES

The United States recently hit an attention-getting milestone, passing China as the country with the most reported infections. But there is deep uncertainty about whether there really are more US cases, and about how well the figures reflect reality.

Countries vary wildly in testing for the virus and how they report the numbers, and experts say most infections are going undetected. So the publicised national tallies are rough, incomplete pictures that may not be all that comparable.

And that's if countries are forthcoming about their data.

Officials in the US say that China, which has reported more than 82,000 infections, has understated its epidemic. Until this week, the Chinese government excluded those patients who tested positive for the virus but had no symptoms.

China also doesn't say how many tests it has conducted, and doubts have been raised about whether it has tested extensively in Xinjiang, the province where it holds hundreds of thousands of Muslims in indoctrination camps.

Few countries have done aggressive testing. And of course, the more testing there is, the more cases are found.

Japan, with relatively few confirmed cases, has conducted only about 500 tests for every million people, raising fears that the virus might be spreading undetected. Similar concerns have been aired in Britain, whose testing rate - more than 2,400 people per million so far - is low by Western European standards, and includes relatively few health workers.

In contrast, South Korea has tested more than 8,000 people per million, and Norway about 17,000.

In the United States, state and local authorities collect figures from public and private labs, but they can be like apples and oranges: Some labs report the number of tests conducted, some report how many people were tested, and some report only positive test results.

The Covid Tracking Project, run by The Atlantic, has tried to compile all the US numbers and reports more than 1.2 million tests so far, more than 3,600 per million people.

WIDESPREAD TESTING

US President Donald Trump has boasted that more people have been tested in the United States than any other country, though on a per capita basis, many developed countries have done more.

But it matters not only how many people are tested, but also when, and who they are. Once again, countries differ, shaping what the numbers mean.

A few countries, like South Korea, Australia and Singapore, got serious about mass testing early on. They used the information to do ambitious contact tracing - finding and testing those who had recently been near infected people, even if they had no symptoms.

That provided a pretty full picture of the outbreak while the numbers were still manageable, and made it possible to slow it down.

Though it did not act on the same scale as those countries, Germany did more testing and contact tracing in the early going than most of Europe.

But most nations with large numbers of cases have done less testing, waited longer to do it in bulk, and made little attempt at contact tracing. They find themselves playing catch-up with the virus, ramping up testing after their outbreaks had already mushroomed.

They detect more cases, but by then it's hard to tell how much of that growth is the expanding epidemic and how much is expanding surveillance. Unable to meet the demand, they often limit testing to the sickest patients and health workers.

FATALITY RATES

It has been stated time and again: Italy and Spain have high mortality rates among coronavirus patients, Germany's is low, and China's is somewhere in between.

It may not be that simple. Counting the dead is as flawed and inconsistent as counting the infected.

Recent reports say that mortuaries in Wuhan, China, where the disease was first discovered, have ordered thousands more urns than usual, suggesting a much higher death toll than the city's official count, 2,535.

The outbreaks in Wuhan, and parts of Italy and Spain, overwhelmed hospitals, forcing many sick people to ride it out at home. No one knows how many people have recovered or died without ever being tested.

Italy and France have reported death tolls that generally included only those who died in hospitals. In Germany, even some of those patients are excluded, because post-mortem testing for the virus is not standard in hospitals.

And if only the sickest patients are tested, then the number of infections will appear smaller and the percentage who die will seem higher.

Germany's low apparent fatality rate - about 1 per cent of those infected - may stem partly from its broader testing of those who are healthy or who have mild to no symptoms, and its narrower testing of the dead.

THE PEAK

Officials often talk about when the epidemic peaks or plateaus - when a country "flattens the curve". But they rarely specify, the peak of what? And how can we be sure we're past it?

When an outbreak is growing unchecked, more people become infected and more die each day than the day before. Italy went from reporting a few hundred newly detected infections per day in early March to more than 6,500 on March 21.

That acceleration cannot continue indefinitely, and more importantly, Italy has strengthened social distancing, apparently slowing transmission of the virus. Since March 21, new confirmed infections have varied between about 4,000 and 6,000 daily. Despite the problems with the available figures, and the dangers of drawing conclusions based on just a few days, it seems clear that after almost two weeks that Italy has passed a turning point.

On a graph, the curve showing the daily count of new cases has gone from rising sharply to moving sideways - the curve has flattened - and even begun to move downward.

That is one corner being turned: The rate of the spread of the virus has slowed down. It takes longer to turn another: the rate of people dying. But that, too, appears to have levelled off in Italy, fluctuating around 800 a day in the last week.

But even when those curves flatten, the epidemic still has not "peaked" by another crucial measure: the number of active cases. That figure continues to rise until the number of patients who either die or recover each day is larger than the number of new infections.

LOCKDOWNS

More than two billion people, including most Americans, are living under something usually called a lockdown. But there is no set definition of that word - or related terms like stay-at-home mandates and social distancing - so the details differ from place to place.

The lockdowns have varying exceptions for certain lines of work, personal circumstances or exercise. Some allow gatherings of up to 10 people, or five, or forbid groups of any size; some exempt funerals, others do not.

The biggest differences may be in enforcement. Some places, like those in the United States with lockdowns, mostly rely on people to follow the rules without coercion.

But Italy and others have deployed soldiers to ensure compliance, and French police have fined hundreds of thousands of people for violating restrictions. China, in addition to using security forces, mobilised an army of volunteers, ratcheting up social pressure to obey.

A few weeks ago, a person could travel around Italy for a valid work or family reason. Now, people are fined for non-essential walking too far from their homes.

But each stage was widely called by the same name: lockdown.