In a remarkably calm and candid tone, Singapore’s prime minister acknowledged something difficult: The new coronavirus disease, Covid-19, was likely spreading in his country, even among people who have never been to China.

“In the last few days,” Lee Hsien Loong said on February 8 in a Facebook video, “we’ve seen some cases which cannot be traced to the source of infection. These worried us because it showed that the virus is probably already circulating in our own population.” If the virus is even more widespread, he added, “it’s futile to try to trace every contact.” Instead of trying to contain the disease, health officials in Singapore will have to accept that it’s gone everywhere and shift to mitigating its impact.

With Singapore’s proximity and close ties to China, it is uniquely exposed to Covid-19. (To date, it has recorded 67 cases of the new coronavirus disease — one of the highest tallies outside mainland China — and no deaths.)

But global health experts increasingly warn that more countries may already have — or will likely see — what Singapore has: broader spread of this new disease within their borders, or “community transmission,” in public health speak.

“Given that the disease is so extensive in China and given that China has so much interaction with the rest of the world … and it appears mild cases are capable of spreading, we should be preparing for and expecting community spread around the world,” Tom Inglesby, director of the Center for Health Security of the Johns Hopkins Bloomberg School of Public Health, said.

This warning comes even as the vast majority of the 64,000 cases and 1,300 deaths of Covid-19 are still found in mainland China.

“The situation in China is quite serious, with multiple generations of sustained transmission,” Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, told Vox. “That’s how you get the makings of [a] global pandemic — if that spreads throughout the world.”

The documented cases of community spread in other countries outside of China have been limited to date, but there’s good reason to believe they’re just the tip of the iceberg. And once more cases are confirmed, the threat of a pandemic (the disease spreading around the world) increases, and so does the number of people the virus will infect and the length of the battle against it. A look at the small outbreaks in five countries — with strong public health systems like the US — helps explain why.

Numerous countries are already seeing limited spread of the coronavirus among people who have never been to China

Source: Johns Hopkins University Center for Systems Science and Engineering

As of February 14, there were 505 cases of Covid-19 in 24 countries outside of China, according to the World Health Organization.

More and more, these include examples of “secondary transmission” of the virus: people who haven’t traveled to China getting sick from someone who has traveled there, or, in the case of Singapore, from an unknown source. Sometimes, these people pass on the virus to others, what’s known as tertiary spread. And, again, this is happening in places with some of the strongest disease surveillance and public health systems in the world.

Here are a few notable examples:

A British national named Steve Walsh stopped for a skiing holiday in the French Alps after attending a business conference in Singapore, where he picked up the virus. By February 7, four adults and one child were confirmed to have the virus after sharing a chalet with Walsh in France ’s Auvergne Rhône-Alpes region. When he went back to England , Walsh infected five others and one person who traveled to Majorca, Spain . Note that Walsh never visited the outbreak’s epicenter, Hubei province in China, but he managed to infect 11 other people.

stopped for a skiing holiday in the French Alps after attending a business conference in Singapore, where he picked up the virus. By February 7, four adults and one child were confirmed to have the virus after sharing a chalet with Walsh in ’s Auvergne Rhône-Alpes region. When he went back to , Walsh infected five others and one person who traveled to . Note that Walsh never visited the outbreak’s epicenter, Hubei province in China, but he managed to infect 11 other people. In late January, a Bavarian businessman in Germany was diagnosed with Covid-19 after he attended a training event with a colleague who was visiting from Shanghai and had the virus but didn’t yet know it. After that, three more employees at the company tested positive for the virus. By February 13, there were 14 patients with Covid-19 in Bavaria: Nine are colleagues who work at the same company as the first German patient, and four are family members who live in the same household as these people. Like those in England and France, German health officials are following up with the contacts of the infected to see if the virus has spread further.

was diagnosed with Covid-19 after he attended a training event with a colleague who was visiting from Shanghai and had the virus but didn’t yet know it. After that, three more employees at the company tested positive for the virus. By February 13, there were 14 patients with Covid-19 in Bavaria: Nine are colleagues who work at the same company as the first German patient, and four are family members who live in the same household as these people. Like those in England and France, German health officials are following up with the contacts of the infected to see if the virus has spread further. In numerous countries outside China — including Japan, France, and the UK — public health workers and doctors treating patients with the virus have been infected. (In China, the situation is much graver. On February 14, China announced its health worker toll for the first time: 1,716 have contracted the virus, and six have died.)

Japan , which quarantined a cruise ship floating on its waters after an 80-year-old man tested positive for the virus, now counts 218 people infected out of 713 tested abroad. That’s the largest outbreak outside of China, and it shows just how transmissible this disease can be.

, which quarantined a cruise ship floating on its waters after an 80-year-old man tested positive for the virus, now counts 218 people infected out of 713 tested abroad. That’s the largest outbreak outside of China, and it shows just how transmissible this disease can be. Then there’s Singapore: More and more of the infections there are originating in Singapore, and as Prime Minister Lee Hsien Loong said, they’re starting to find cases that can’t be traced back to others.

According to the WHO, as of yesterday, 22 percent of the cases outside China have involved secondary or tertiary transmission, and of those, eight cases cannot be linked to known transmission chains.

“The detection of this small number of cases could be the spark that becomes a bigger fire,” WHO Director General Tedros Adhanom Ghebreyesus said this week. “But for now it’s only a spark.”

Why this may be the tip of the iceberg

Of the 15 cases of coronavirus in the US, 13 involved travelers in China and two were very close contacts of those travelers. “We have not had community spread from person to person,” Fauci said. “Hopefully we will not — but if many countries start having [community] transmission and get beyond the ability to contain it, it’ll be very difficult to prevent the global spread. You can’t inhibit travel from every country.”

And yet, it’s difficult to know for sure whether there’s what Tedros called “a bigger fire” already. Here’s why the cases that have been detected outside China may be the tip of the iceberg:

Countries are still mostly looking for the disease in people from China: The main method of screening for the disease in many countries is still testing passengers from China, or from Hubei province only. But as we’ve seen, spread is happening beyond just people who’ve traveled recently in Hubei. And people carrying the virus from elsewhere may be undetected. “What happened in the UK, where a cluster of infections was started by someone who was infected outside of China, could happen in the US,” Jennifer Nuzzo, an infectious disease expert and senior scholar at the Johns Hopkins Center for Health Security, told Vox. “I’m not confident we’d catch it before it spread on, since we’re using severity of symptoms and history of travel to China as criteria for testing people for this virus.”

“There’s a high probability that we’ll see community spread in the US,” she added.

Though the US Centers for Disease Control and Prevention maintains that the risk of coronavirus spread on the home front is low, it’s beginning to change its screening strategy to look for people with the virus who aren’t returning travelers from China. The CDC will use the national flu surveillance tracking infrastructure to test patients who have flu symptoms for Covid-19 in five cities across the US. “I don’t think we [have additional cases] right now but I think we can in a period of time,” Fauci added. “That’s why we are doing the screening in five cities. But if it wasn’t possible [there are additional cases to find,] we wouldn’t be doing it.”

Many countries are only now getting diagnostic tools up and running: Until this week, only two countries in Africa had the lab capacity to screen for this disease — Senegal and South Africa. While other countries are now scaling up, this outbreak has been going on since late last year, and it’s possible cases have gone uncounted.

So far, not a single case has been detected in Africa — and Africa is thought to be at particular risk given its links to China, with more than a million Chinese workers. Same goes for other lower-resource countries, like Indonesia, which also haven’t found cases.

Some people may present with abdominal pain before respiratory symptoms — and that’s not something health officials are screening for: This coronavirus is still very new, and we don’t know the entire spectrum of illness yet, but we’re learning the disease may sometimes surface in surprising ways. Though it’s a respiratory infection, a recent JAMA article found that people can present in the hospital with abdominal symptoms such as discomfort first. This means “we may not be detecting cases that do not present in the classic way with fever and respiratory symptoms,” said William Schaffner, an infectious disease expert at Vanderbilt University.

“The challenge with this illness is that the clinical symptoms resemble other viral illnesses, like flu,” Inglesby added.

Putting all this aside, models have repeatedly suggested there are thousands more cases than have been detected. According to a new model published in The Lancet by academics at the University of Hong Kong, 75,000 people may have been infected in Wuhan alone as of January 25. That was more than two weeks ago — and only 64,000 cases have been found to date.

That’s why Michael Ryan, executive director of WHO’s health emergencies program, said in a press briefing on February 13, “We have work to do to see how big that iceberg is.”

Don’t panic; prepare

To understand Covid-19’s spread, scientists need what’s known as serology testing, Ryan said. With serology tests, health officials can find out how many people developed antibodies to fight the virus by randomly sampling populations. This would detect all those mild or even asymptomatic cases that may be getting missed right now and help determine how deadly the new virus really is (as I’ve explained).

For now, it’s important to remember that a disease can spread widely, and even become a pandemic, and not be particularly severe. In his Facebook video, Prime Minister Lee repeatedly asked Singaporeans not to panic, reminding them that this virus already appears to behave more like seasonal flu than SARS: While it’s more contagious than SARS, it looks much less deadly — a point Fauci recently reaffirmed.

At the same time, even a less severe pandemic has the potential to overwhelm a country’s health infrastructure. There are studies out of China suggesting as many as a quarter of patients need care in the ICU, for example — something that could wreak havoc on health systems and cost billions. “There’s always chance [this disease could] overwhelm health systems in a number of countries, even in countries with good health systems,” Fauci said.

“We don’t have enough data to know what the burden will be in the US,” Inglesby added, “but there does seem to be enough information that hospitals and public health agencies should be planning for a potential rise in very sick people that will need critical care in the months ahead.”