Hours after Nigeria confirmed its first case of coronavirus Friday, Olumide Makanjuola, who lives in the state of Lagos, opened WhatsApp and was bombarded with a “sense of panic.”

Users on the messaging service had copied, pasted and forwarded notes warning that local flights, hotels and schools might have been contaminated. None of the information had been verified, Makanjuola said, but multiple versions of it snaked their way through private WhatsApp groups, some with hundreds of participants.

“The virus is closer to us than we think,” two of the messages ominously concluded.

As government leaders and health professionals race to contain an outbreak on the verge of a pandemic, they are simultaneously battling another hard-to-defeat scourge: the explosion of half-truths and outright falsehoods online. Nowhere is the threat more dire than on WhatsApp, a service largely hidden from public scrutiny, vast in its global reach and often at the center of some of the world’s most panic-inducing conspiracy theories.

People in Nigeria, Singapore, Brazil, Pakistan, Ireland and other countries say they’ve seen a flood of misinformation on WhatsApp about the number of people affected by coronavirus, the way the illness is transmitted and the availability of treatments. The messages and voice memos have instilled fear, troubled businesses and created public health headaches for governments, including Botswana, which pleaded with people last month to be wary of what they’re reading and sharing on the service.

WhatsApp’s parent company, Facebook, has labored to vet or remove similar harmful coronavirus-related content on its namesake social network. But the tech giant can’t monitor WhatsApp the same way. Conversations there are encrypted, meaning messages can be read only by the sender and recipients — not the company, even in the midst of an urgent, deadly international health crisis.

Makanjuola, a sexual-health and rights advocate, said he appreciated the delicate balance between security and freedom of expression that made oversight of WhatsApp so difficult. But, he said in an interview, “the idea people can transfer panic is something that really does worry me.”

Carl Woog, a spokesman for WhatsApp, said the company had been vigilant in trying to reduce the reach of misinformation, stressing it is working with governments and others “to respond to the immense challenge presented by the Coronavirus.”

“WhatsApp is an important tool for health workers to coordinate and we have engaged health ministries around the world to provide simple ways for citizens to receive accurate information about the virus,” he said in a statement.

With 2 billion users globally, WhatsApp has emerged as one of the world’s most popular messaging systems. Users flocked to the service in its early days, when it cost only a dollar each year, because it was much cheaper than texting over fee-heavy traditional phone services. Now free, it ranks as one of the top downloads throughout Europe, Africa and Asia, particularly in countries where telecom infrastructure is lacking.

Like Facebook, though, the meteoric rise of WhatsApp also has brought myriad unanticipated challenges globally. Malicious actors at times have seized on WhatsApp’s expansive audience — and its easy tools to share text, audio, photos and videos among groups of friends — to push harmful messages to large numbers of people.

In India, viral misinformation on WhatsApp triggered real-world violence. At one point in 2018, experts could trace two dozen deaths to fake news, including false reports of child kidnappings, that spread on the service. And in Brazil, WhatsApp has served as a staging ground for political falsehoods, including a raft of rumors that imperiled the country’s most recent election.

Under global pressure, WhatsApp has labored to improve its digital defenses — labeling messages that had been shared, for example, while restricting the number of times content could be forwarded to others. Researchers say some of the technical tweaks have helped by introducing what they call friction, slowing the proliferation of propaganda, conspiracy theories and other harmful content.

Woog, the spokesman, added that WhatsApp had run local education campaigns around the world “to help encourage people not to spread rumors.” WhatsApp also bans millions of accounts each month for sending bulk or automated messages.

But WhatsApp’s prized security features also prevent the company from moderating content with the same aggressiveness as public-facing social-networking services, which have invested heavily in workers and technology to protect users from abuse. The lack of visibility has frustrated regulators, including in the United States, where Attorney General William P. Barr said WhatsApp stymies the investigation of crimes, including child sexual exploitation. Authorities in the United States and around the world have threatened to force Facebook to give governments access to encrypted data.

Even if WhatsApp could scan every message, users might bristle to learn a profit-seeking tech giant inserted itself into their private, sensitive conversations. The result is that the very features that make WhatsApp so alluring to people — and so important to Facebook — also heighten the likelihood that it could become a breeding ground for misinformation.

In recent weeks, WhatsApp users throughout Africa and Asia reported a stream of text messages and voice memos in private channels that pitch fake coronavirus cures, according to copies of the messages annoyed recipients shared with The Washington Post and published on sites such as Twitter. Some of the recirculated texts wrongly list garlic, salt water and a type of tea as natural remedies for the outbreak, even though no treatments exist.

Other problematic messages inflate the number of people affected or push widely debunked conspiracy theories about the coronavirus and its origins. Often, users pass along these hoaxes — even if unwittingly — in groups of contacts they create. These groups are often capped at 250 members, but one falsehood shared in multiple channels still can become a contagion of its own.

Tolu Ogunlesi, a top new-media adviser to Nigerian President Muhammadu Buhari, said he has seen a wave of conspiracy theories about coronavirus metastasizing this way. One alarming WhatsApp message he shared with The Post said falsely that a major oil company in Banana Island, Lagos, had been placed on lockdown after a worker had contracted the coronavirus.

Misinformation is hardly a new phenomenon in Nigeria: In 2018, a hoax that Buhari died and had been replaced by a clone went viral online, including on WhatsApp, which Ogunlesi described as the most popular communications platform in the country.

Nigerians generally have become more digitally savvy, with an “increasing awareness now about the ubiquity and dangers” of misinformation, according to Ogunlesi. But he still has “started policing” WhatsApp groups, he said, trying to push back on fake news and encouraging his peers to do the same.

“I’m not quite sure how much they can do in terms of actually policing WhatsApp content the way they police Facebook content,” Ogunlesi said of the tech giant. “If I had a message to the tech companies, it would be to invest a lot more money in digital literacy.

“Everybody is dealing with this,” he continued. “This is a global thing, and nobody has quite figured out what needs to be done.”

The government of Botswana warned people about WhatsApp last month. “The Ministry notes with concern the misleading information circulating on social media particularly Facebook and WhatsApp, about #coronavirus situation in the country,” local authorities tweeted on Feb. 2. The Embassy of Botswana in Washington did not respond to requests for comment.

Government officials in Brazil — long aware of the power and pain of WhatsApp — similarly sounded an urgent note of caution. The state’s Ministry of Health tracked and vetted inaccurate WhatsApp messages about the number of citizens who are sick, along with other hoaxes, including one that wrongly said Chinese imports are tainted with the illness.

“False information causes panic in the population and hinders the investigative work of the competent authorities,” the government warned. A spokesman for the Embassy of Brazil in Washington did not respond to requests for comment.

Hoping to help governments grappling with the coronavirus, WhatsApp said it has offered help to regulators in 15 countries so far, including with efforts to set up dedicated tip lines on the service for accurate health information. In Singapore, where regulators just last week warned about “unsubstantiated information over WhatsApp,” health officials now are able to pass along daily, credible updates to anyone who signs up.

But the battle against rumors and hoaxes is difficult to wage, especially in real time, as businesses and users have learned in recent weeks. A digital concierge for the Conrad hotel in Dublin spent at least some portion of Friday telling people it was not in fact shuttered because of a suspected case of coronavirus on the premises. Myths about local hotel lockdowns appeared to originate in online channels, including Twitter and WhatsApp, social media complaints suggest.

“There are no confirmed cases of coronavirus, and the hotel is operating as usual,” said John Brooks, a spokesman for Hilton, which owns the facility. The company declined to comment on the role of WhatsApp in stoking rumors.

Other WhatsApp users are fighting back.

In Karachi, Pakistan, local aviculturist Urooj Zia said she saw misinformation last week in an unlikely place — WhatsApp groups dedicated to parrot breeders. The dubious messages said nearby Aga Khan University Hospital had released a patient after a “false indication of the virus.”

The text she received, and shared with The Post, concluded: “Don’t panic. Everything is under control.”

A policy researcher by background, Zia told The Post she learned from doctors that no such patient had been discharged. Still, she said two people had circulated the message in two different groups, each with hundreds of followers, raising the potential that it spread even further.

“Debunking crap online,” she said, “is what I do because someone has to.”‘