The world marked a grim milestone on Thursday, registering more than 1 million confirmed cases of the deadly coronavirus that has swept the globe in less than five months.

But in reality that mark — 1,002,159 around 4 p.m. EDT — was crossed much earlier.

That's because the number of official cases, compiled by Johns Hopkins' Coronavirus COVID-19 Global Cases website, are only those identified through testing. Cases not tested would include asymptomatic individuals; people who may have died of complications of the virus without anyone knowing it; and those whose symptoms were not serious enough to qualify for testing.

"The million (cases) is clearly way under what the actual number will be because of all the issues of testing and all the people with mild symptoms that haven’t been tested," said Dr. Steven Corwin, president and CEO of NewYork-Presbyterian Hospital.

He said the U.S. figures are especially underreported "because of the lag that we had getting testing underway and the ability to only test the sickest of patients to begin with."

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That is an especially alarming reality because people with undetected cases unwittingly spread the virus, especially within families or if people mix in large, public gatherings.

"Every infectious agent only goes as the hosts go," said Dr. Ogbonnaya Omenka, an assistant professor and public health specialist at Butler University's College of Pharmacy and Health Sciences. "In essence, our social patterns are excellent indicators of how far and wide an outbreak would go, if they remain unchanged. This is why physical distancing has been put in place, to throw the virus off-balance, so to speak, by breaking its chain of transmission."

The U.S., with more than 236,000 cases as of Thursday, tops the list of countries with the most infections, followed by Italy and Spain with just over 110,000 each. China has fallen to fourth, with just under 82,500 cases, according to Johns Hopkins COVID-19 case tracking system.

Corwin said New York City's burgeoning caseload mirrors what unfolded in Italy, which has seen the most deaths worldwide. "I’m fearful that in the rest of the country we’ll see that coming in waves," he said.

That suggests that the worldwide death toll – which stands at just more than 51,000 Thursday – also will set milestones, particularly as the virus spreads in the U.S.

The outbreak, Omenka said, normally will stop "when it runs out of susceptible hosts, once already infected persons start developing immunity against the agent, or a vaccine becomes available."

The key, he said, is how much the public follows guidelines keeping them apart.

"It cannot be overemphasized the importance of not only public cooperation, but the need for a nationally coordinated response to this outbreak, because it would be horrible if some states experience a resurgence of the disease after overcoming it, due to outbreaks in other states," he said.

The patchwork nature of the U.S. response was underscored by widely divergent assessments in individual states.

While the governors of Illinois and New York issued stay-at-home orders for residents as early as March 20, Florida Gov. Ron DeSantis, despite heavy criticism, did not mandate such measures until this week, despite 7,000 confirmed cases and 85 deaths in his state as of Tuesday. Georgia and Mississippi were the last holdouts in the South.

The virus, which apparently started in a market in Wuhan, China, in December 2019, has now reached 180 countries or regions.

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Research suggests that the new coronavirus, like its cousins SARS and MERS, has its origin in bats. Scientists suspect the virus was initially transmitted to another animal – an "intermediary host" – before it spread to humans.

The rate of spread varies from country to country

South Korea reacted quickly and aggressively, applying a well-organized and widespread testing program to locate carriers, trace their contacts and quarantine their contacts. It has 9,976 confirmed cases and 169 deaths as of Thursday.

According to the Worldometer website, which tracks the data from official sources on testing, South Korea — with a population of almost 52 million — has tested more than 270,000 people, which amounts to more than 5,200 tests per million inhabitants.

The United States, by comparison, has carried out just over 1.16 million tests, according to the COVID Tracking Project, or about 3,547 tests per million inhabitants.

After months of quarantine measures and travel restrictions, China's cases have fallen dramatically. China's National Health Commission says that all the new cases are being imported from abroad. Recently, Chinese officials lifted travel restrictions on more than 60 million people in the Hubei province, where the outbreak originated.

What are symptoms of the coronavirus?

Symptoms can range from mild to severe, and some people don't have any symptoms. According to the CDC and the WHO, symptoms include:

Fever

Dry cough

Shortness of breath

Tiredness

Some people also develop aches and pains, nasal congestion, runny nose, sore throat or diarrhea

Symptoms may appear anywhere between two to 14 days after exposure, with the average patient seeing onset at around five days, according to the CDC.

But details of the most common symptoms are still evolving. One 66-year-old New York neurosurgeon, Ezriel Kornel, who tested positive for the virus didn't initially have any of the most common symptoms. Newer reports are also suggesting that a loss of a sense of smell or taste may be a symptom of COVID-19.

When should you get tested?

If you have symptoms and want to get tested, the CDC recommends calling your state or local health department or a medical provider.

At this time, the CDC recommends that clinicians prioritize testing hospitalized patients and symptomatic healthcare workers. Second-level priority includes patients in long-term care facilities with symptoms, patients 65 years of age and older with symptoms, patients with underlying conditions with symptoms and first responders with symptoms.

Not sure if you should get tested? The CDC website features a "self-checker" to help you make decisions about seeking medical care. The feature is not intended for the diagnosis or treatment of COVID-19 and is intended only for people in the U.S.

Contributing: Ryan Miller, Jesse Yomtov and Grace Hauck