5 things to know about monkeypox after a new case appeared in U.S.

For the second time this year, the United States has an imported case of monkeypox. A traveler from Maryland who had recently returned from Nigeria has been diagnosed with the dangerous illness, the Centers for Disease Control and Prevention said Wednesday.

The unidentified person is in isolation in Maryland, the CDC said in a statement. The Maryland Department of Health said in a statement that the individual has mild symptoms and is not in the hospital.

The earlier U.S. case this year occurred in Texas in July, also in a person who had traveled to Nigeria. There were no secondary cases from the Texas patient, though more than 200 people who had contact with the individual were monitored.

The CDC said it is working with the airline on which the passenger traveled, as well as state and local health authorities in the Washington, D.C., area to identify other passengers and people who may have been in contact with the infected person.

But the agency said it believes the risk of transmission during travel will have been lowered because people on flights are currently required to wear masks.

In recent years there have been a number of reports of exported monkeypox cases cropping up around the globe. Here are some facts about this rare disease:

The virus

Monkeypox is caused by a virus that is related to smallpox; both are Orthopox viruses. Smallpox, once a common scourge, was declared eradicated in 1980.

Its name suggests it comes from monkeys, but that in fact is not the case. While the first time the virus was seen to cause an outbreak was in 1958, in a colony of research monkeys, the true reservoir of the virus remains unknown. A number of African rodent species are known to be susceptible to the virus and have been seen to be involved in its transmission. (More on this in a bit.)

The disease in humans

The incubation period for the disease — the time from exposure to the onset of illness — ranges from five to 21 days. People who are infected initially develop mild, flu-like illness — headache, fever, chills, and swollen lymph nodes. But a few days later, a rash will appear, often starting on the face. The rash will typically spread to other parts of the body, though mainly the extremities. Palms of the hands and soles of the feet are frequently affected.

Scarring lesions will form in a stage of the disease that can last between two and six weeks.

The disease can be deadly. In Africa, monkeypox has been fatal in about 1 in 10 cases, with severe disease and death more likely among children.

Spread to and among people

The virus transmits to people from infected animals, entering through cuts in the skin, the respiratory tract, or the mucous membranes around the eyes or in the nose and mouth.

A large outbreak in the United States in 2003 — the first time monkeypox was reported outside of Africa — saw 47 confirmed and probable cases reported from six different states. The outbreak was linked to infected exotic pets imported from Ghana, which in turn infected some prairie dogs sold as pets.

Person-to-person transmission can occur, and is thought to occur mainly through virus-laced droplets. But direct contact with lesions or bodily fluids from an infected person, or indirect contact via contaminated clothing or linens, can also result in transmission.

Where it is found

The virus appears to be present in a belt of countries in West and Central Africa, with locally acquired cases reported from Sierra Leone, Liberia, Côte d’Ivoire, Nigeria, Cameroon, Gabon, Central African Republic, the Republic of Congo, and the Democratic Republic of the Congo.

The United States, the United Kingdom, Israel, Benin, South Sudan, and Singapore have reported imported cases of monkeypox.

An unintended consequence of smallpox eradication

The WHO suggests that the risk of contracting monkeypox in countries where it is found may be greater in people middle-aged and younger — people who were not vaccinated against smallpox in childhood.

The eradication of smallpox led to the termination of routine smallpox vaccination around the world, which may have contributed to the increase in human cases in Nigeria that has been observed since 2017, Australian scientists suggested in a paper published earlier this year.