More than 200 people in the US are being monitored after they were potentially exposed to monkeypox.
The individuals - who are spread across 27 states - were in contact with a US resident who contracted the disease in Nigeria before landing back in their home country on 8 July.
The infected traveller was then hospitalised in Dallas, Texas, on Thursday (15 July), where they received their diagnosis.
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Monkeypox belongs to the same family of viruses as smallpox and the strain the person is infected with kills about one in 100 who are infected, according to The United States Centers for Disease Control and Prevention (CDC).
Those who are being monitored include those who were sat within six feet of the infected individual on the Lagos to Atlanta flight, as well as passengers who used the same toilet, flight attendants, cleaners and some family members in Dallas.
Andrea McCollum - who leads the poxvirus epidemiology unit at the CDC's National Center for Emerging and Zoonotic Infectious Diseases - told US health news website Stat: "It is a lot of people.
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"We define indirect contact as being within six feet of the patient in the absence of an N-95 or any filtering respirator for greater than or equal to three hours.
"We're in the time frame where we certainly want to closely monitor people."
The CDC is hopeful that the rare but potentially serious illness was less likely to be transmitted on the flight due to current Covid-19 regulations.
In a statement, the CDC said: "The Centers for Disease Control and Prevention and the Texas Department of State Health Services confirmed on 15 July a case of human monkeypox in a US resident who recently travelled from Nigeria to the United States.
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"The person is currently hospitalized in Dallas. CDC is working with the airline and state and local health officials to contact airline passengers and others who may have been in contact with the patient during two flights: Lagos, Nigeria, to Atlanta on 8 July, with arrival on 9 July; and Atlanta to Dallas on 9 July.
"Travelers on these flights were required to wear masks as well as in the US airports due to the ongoing Covid-19 pandemic. Therefore, it's believed the risk of spread of monkeypox via respiratory droplets to others on the planes and in the airports is low."
Monkeypox is a rare but potentially serious viral illness that typically begins with flu-like illness and swelling of the lymph nodes before progressing to a widespread rash on the face and body.
Most infections last two to four weeks.