DAKAR, Senegal — The first confirmed case of coronavirus in sub-Saharan Africa has emerged in Nigeria, the continent’s most populous country and biggest economy, sparking fears the contagion could spread quickly and stymie business.

Anxiety blazed across Nigerian social media Friday after authorities reported an Italian man was receiving treatment at a hospital in the commercial capital, Lagos — at 21 million people, Africa’s largest city.

The man, who is in his 30s and works for a Nigerian concrete company, had traveled northwest this week to a factory in Ogun state before he sought help for a fever, health officials said.

Now, investigators are tracking down the other passengers who arrived on his Turkish Airlines flight from Milan.

“The most important work today is contact tracing,” said Chikwe Ihekweazu, chief executive of the Nigeria Center for Disease Control, who added that investigators have collected a “detailed itinerary” of the man’s time in Nigeria.

Markets tumbled Friday as the World Health Organization raised its global risk assessment of the coronavirus to “very high,” dashing hopes the new virus could be largely contained to China.

More than 83,000 cases have been confirmed in 50 countries, officials say, and more than 2,800 people have died.

Although 36,000 patients have recovered, WHO officials have expressed concern the disease could overwhelm countries across Africa and South America.

“Our greatest concern is about the potential for spread in countries with weaker health systems,” WHO Director General Tedros Adhanom Ghebreyesus said in a January speech.

To date, 26 African nations have reported suspected coronavirus cases, according to the Africa Centers for Disease Control and Prevention. Egypt and Algeria have also confirmed cases.

Governments have scrambled to boost their preparedness, but more specialized medical training and equipment is needed, officials say, to address the highly contagious respiratory threat.

Nairobi, the Kenyan capital, is ready for only a dozen patients in its isolation ward, a health ministry official said Thursday at a news conference, but a larger space is expected to be finished in about a month.

Kenyan President Uhuru Kenyatta ordered that expansion to be completed within a week.

A ban of flights between Kenya and China, the East African nation’s biggest trading partner, was reinstated Friday by judges at a high court, who also requested that all 239 passengers who arrived in Nairobi on a flight this week from Guangzhou be tracked down and put into quarantine at a Kenyan military facility.

Nigerian health officials projected confidence Friday as they sought to soothe fears.

“Nigeria is the most populous country, and Lagos is definitely the most populous city, but we also are the most resourced,” said Ihekweazu, the Nigerian CDC head.

Buildings the Italian businessman visited have been placed under quarantine, authorities said, and those who interacted with him have been instructed to undergo daily temperature checks for two weeks.

Obafemi Hamzat, deputy governor of Lagos state, said in a televised interview that Lagos had beds ready for 100 people in an isolation ward if the virus spread.

“There is only one bed that is occupied today,” he said.

Since the news broke, Matthew Alo, a 25-year-old banker in Lagos, has been avoiding crowds and regularly dousing his hands in sanitizer.

“I’m staying aware of my surroundings,” he said. “The virus is deadly.”

When Ebola swept across West Africa in 2014, ultimately killing more than 11,300 people, Nigeria swiftly contained its outbreak.

Only 20 people were diagnosed with the virus, eight of whom died.

Officials say lessons from that time shape the urgent response to the coronavirus threat: The first patient diagnosed with Ebola in Nigeria, who arrived in the country from Liberia, went quickly from the airport in Lagos to a hospital, which helped limit his contact with the community.

The Italian man, however, showed no signs of illness when he landed, officials say, so he could have exposed more people to the virus during his trip.

Tomi Coker, the health commissioner in Ogun state — the manufacturing hub where the patient worked — called on people with a fever or a bad cough to contact doctors in the area.

“As a state, we are prepared,” she said, “and we will continue to prepare.”

Bearak reported from Nairobi. Siobhán O'Grady in Washington and Borso Tall in Senegal contributed to this report.