It started as a mysterious, pneumonia-like illness. Cases of it appeared before Christmas in the Chinese commercial center of Wuhan, population 11 million, traced to a wet market there. It presented in infected patients as a fever, shortness of breath, and body aches. Wuhan officials said early on there was no clear evidence it could be transmitted from human to human.

They were wrong.

Such transmission did occur—at an alarming rate. Five days after it got its coronavirus name, Thailand confirmed the first case outside of mainland China. Within weeks, infections dotted nearly every country in Asia.

Now the coronavirus, which causes the disease COVID-19, has reached the new, ominous milestone of 100,000 global infections, according to Johns Hopkins data.

Wuhan's Hubei Province remains the most affected with 67,600 cases and over 2,900 deaths in total. But the outbreak's ongoing march across the globe has revealed new hotbeds of contagion—first in some of Asia's biggest economies and then in concentrated pockets in Europe and the Middle East. It has exposed borders as flimsy human constructs, underscored the world's vast connectedness, and left a trail of lessons to be learned.

As the coronavirus counter ticks to 100,001, here's a look back at some of the ways it's spread.

Japan and the Diamond Princess

Japan confirmed its first case of coronavirus on Jan. 16. The patient was a man from China who had traveled to Japan from Wuhan. Over the next week, Japan confirmed two more cases, both Chinese nationals who had travelled from Wuhan. But soon there were signs of community transmission, as a case confirmed on Jan. 28 was recorded in a man who had never visited China.

Despite the evidence of community transmission and the high number of Chinese tourists that visit Japan each year, the Japanese government was slow to develop a response plan to the viral outbreak. Tokyo only adopted a “basic policy” to prevent community spread on Feb. 25—nearly a month after the World Health Organization had declared the outbreak a Public Health Emergency of International Concern and three weeks after the Diamond Princess cruise ship docked in the city of Yokohama, issuing Japan its biggest coronavirus challenge.

A former passenger of the Diamond Princess, who had disembarked in Hong Kong on Jan. 25, tested positive for COVID-19 a week later. By this time, the cruise ship was back in Yokohama but due to depart on Feb. 4. That sailing was cancelled so Japanese authorities could screen passengers for the virus; they found ten cases. The cruise liner was put under quarantine, with all 3,700 passengers and crew locked on board.

During the month-long quarantine, the number of infected people on the ship swelled to 706 as Japanese authorities faced criticism for bungling the situation. There were no effective quarantine measures on board the ship to separate infected passengers from the others, rendering the quarantine exercise ineffective.

Most foreign passengers repatriated from the ship were subjected to a further 14-day quarantine after arriving home. In Japan, however, Tokyo allowed Japanese citizens who had not yet tested positive for coronavirus to leave the boat and return home after the minimum 14-days on board.

A number of disembarked passengers later tested positive for coronavirus, having already travelled hundreds of kilometers across the country, risking infection to others along the way. Japan now has 360 cases of coronavirus and, unlike in city states such as Singapore, the route of transmission has become virtually impossible to track.

Singapore

Singapore reported its first case on Jan. 23—a Chinese woman who had flown from Wuhan, the center of the outbreak. Since then, cases have risen to 117; 78 have recovered with no deaths, making the city-state a prime example of how to contain the coronavirus. However, Singapore’s success is not easy to replicate.

The key to tackling the outbreak has been the government’s iron grip on power over its 6 million residents. The strong party rule has allowed authorities to act resolutely with little resistance from the public. Officials have put more than 2,000 people in quarantine, with the government placing them under close surveillance and punishing those who break protocol. A couple was fined for lying about their travel history, and an expat worker was expelled after breaking quarantine.

In a way, Singapore has benefitted from its experience with SARS, or severe acute respiratory syndrome, in 2003 when the city suffered 33 deaths from 238 cases. The tragedy prompted the government to install quarantine facilities in its hospitals and build a dedicated center for infectious diseases.

But despite its swift response and advanced preparation, Singapore still became ground zero for a coronavirus “super spreader” event in late January. An attendee at a business conference hosted by U.K. gas analytics firm Servomex at the Grand Hyatt in Singapore contracted the disease and unwittingly transmitted it to 11 others as he returned to the U.K. via France. Five of the 11 were confirmed infected in France while one traveled back to Mallorca, Spain, before testing positive.

As rumors of the “super spreader” emerged, media outlets raced to identify the individual who’d carried the virus across the globe and back to the U.K., eventually unmasking him as a man from Hove, in South England, where the five remaining secondary infections took place. One doctor from the area has since been diagnosed with the virus, and 30 other doctors have isolated themselves as a precaution.

In a statement, the Hove man asked the media to respect his privacy, an individual right that's been called into question as governments struggle to limit contagion. In Singapore, the government’s ability to track cases and identify points of transmission has been integral to interrupting the disease’s spread—but the state also makes a lot of that data publicly available, including the home and business addresses of patients.

South Korea

Whereas Singapore has deployed mass surveillance to combat the coronavirus, South Korea has turned to mass testing.

South Korea reported its first confirmed case of coronavirus on Jan. 20, but the disease remained mostly contained in the country until Feb. 18, when an outbreak tied to a branch of Shincheonji Church of Jesus, a secretive extreme Christian sect, popped up in the southern city of Daegu.

A 61-year-old woman became known as patient zero of the Shincheonji cluster; she’d attended church services regularly and was known for fostering close physical contact with fellow worshippers. As of March 1, South Korean health officials had traced at least 3,736 confirmed cases to her. In total, South Korea has reported more than 6,500 infections and at least 40 deaths as of Friday.

While the woman had not recently traveled to China, her case exposes how intertwined China and South Korea’s populations are in official and under-the-radar ways. An estimated 1 million Chinese nationals live in South Korea, and there are at least 120,000 South Koreans living in China, according to China’s 2010 census. While religion is officially banned in China, the Shincheonji Church has a branch with hundreds of followers in Wuhan, which at least some Daegu-based parishioners had visited recently.

Rather than impose lock-down style measures as China did, South Korean authorities and biotech companies mobilized an expansive testing operation in response to the crisis, testing hundreds of thousands of people, some via drive-thru style clinics.

South Korea has also cracked down on the Shincheonji church itself. Church leader Lee Man-hee bowed on the ground begging for forgiveness during a press conference on Monday, but authorities are reportedly still pursuing homicide charges against him due to the church hiding information about the outbreak.

Italy

In Italy, the coronavirus has revealed the deep ties between two regions—Lombardy in the country's north, and Zhejiang in China's south, separated by 9,000 kilometers.

In late January, Italian authorities confirmed the first two cases of coronavirus from two Chinese tourists in Rome, but authorities now believe that the two Chinese nationals did not infect anyone else. For weeks, new infections remained low and at the time the Italian Prime Minister Giuseppe Conte said it was because Italy employed the “most rigorous” prevention measures in Europe.

But on Feb. 21, Italy reported that a 38-year-old man identified as Mattia had coronavirus in Codogno, a small town in Italy’s northern Lombardy region. How Mattia became infected remains unclear as he had not recently traveled to China, but he became known as patient zero for triggering a surge in cases across the region. In the week after Italy confirmed Mattia’s case, 888 people were diagnosed with the disease across northern Italy. In response, authorities quarantined towns across northern Italy, banned large gatherings like sporting events, and temporarily closed all schools and universities nationwide.

Italy’s outbreak has also marked some of the first reported cases of coronavirus being imported back to China. Qingtian, a small county in China’s southeastern Zhejiang province, confirmed eight new cases of coronavirus on March 2 from Chinese nationals living in Italy’s Lombardy region. There are more than 300,000 Chinese nationals living in Italy, and roughly 90% of them in the Zhejiang Province; the garment industry there has attracted Chinese workers.

These imported cases prompted Chinese authorities to begin quarantining foreign travelers and urging overseas Chinese populations to stay away in fears of they might lead to new outbreaks.

At nearly 4%, Italy has one of the highest death rates of coronavirus in the world, as 148 of 3,858 infected people have died. Italy has the world’s second-oldest population, and experts suspect that the country’s elderly citizens are particularly vulnerable to fatal infections.

Iran

Just as Italy had touted its low caseload early in the outbreak, officials in Iran also spoke too soon. Just two weeks ago, officials in Iran, praising the country’s health system, predicted the country would not be affected by the coronavirus outbreak. Iran has since become one of the biggest hotbeds of the disease, logging nearly 3,000 cases and 92 deaths.

On Feb. 19, Iran confirmed its first two cases in the city of Qom—and later that day said both people had died. The health minister said the virus arrived in Iran via a merchant who frequently traveled between Qom and China and who was one of the two deaths.

The virus has since spread to at least three more cities, including Tehran, the capital, after officials said they would not put towns under quarantine or close religious pilgrimage sites; they encouraged citizens to go to the polls for parliamentary elections.

By Feb. 24, cases had jumped to 61, according to deputy health minister Iraj Harirchi. At the press conference to announce the figures, Harirchi's own condition became a focus. As he downplayed the virus’s spread, Harirchi coughed into his hand and removed his glasses to wipe sweat from his brow so often that at one point someone stepped onto the podium to hand him a new box of tissues.

The next day, Harirchi tested positive for COVID-19.

Around 8% of Iran’s members of parliament have contracted the virus, and a senior advisor to Iran’s supreme leader died of the disease on March 2. The nation now has more than 3,500 cases; at least 107 people have died.

Some schools and universities are closed, but major religious sites remain open, a move one doctor described as "criminal." (Iran has halted Friday prayers in all provincial capitals this week to control the spread of the outbreak.) Medical staff say they have been told to keep quiet about the outbreak. One lawmaker called official numbers “a joke” and said in reality the infection and death counts are much higher.

Iran's health ministry said on Thursday the government had approved several bills aiming to contain the outbreak, and said the WHO approved of the ministry's plans, which include setting up checkpoints to limit travel between major cities and encouraging the public to limit the use of paper money.

Some see Iran’s early opposition to restricting travel from China as government reluctance to jeopardize its relationship with its largest trading partner; U.S. sanctions may be contributing to a medical equipment shortage as well. Iran has become a sort of ground zero of the virus in the Middle Ease, with most of the new cases in the region linking back to Iran.

A 'democratic' virus

That the coronavirus has reached so far—to least 84 countries—so fast is alarming. "This is literally like trying to stop air," New York Governor Andrew Cuomo said Wednesday, as the state confirmed five new cases.

Governments have tried to tame the outbreak, in part, by limiting who can cross what border. And while some of those measures have proven wise in hindsight, the coronavirus has repeatedly defied city lockdowns and countries' flight restrictions.

In a selfie video posted after his COVID-19 diagnosis, Harirchi, the deputy health minister in Iran, said the coronavirus is "democratic."

"It does not distinguish between poor and rich," he said.

Nor it distinguish between geographies. And in that sense, its nature confounds the current political climate. In an era of deepening isolation and nationalist sentiment—of walls being built and blocs being broken—it is truly a global problem, still desperately awaiting a global solution.