The U.S. Centers for Disease Control and Prevention issued a health alert on Monday for a more dangerous strain of mpox, also known as monkeypox, that was detected in the United States in recent days.

In a health alert, the CDC said that the first case of clade I mpox was discovered in the United States in California after the infected person “had recently traveled to areas experiencing clade I monkeypox virus.”

No additional mpox cases in the United States have been discovered as of Nov. 18, according to the CDC alert. Local and state health agencies are still investigating additional contacts.

The infected person has displayed “relatively mild illness and is recovering,” which the CDC said is “consistent” with recent clade I mpox cases.

Health authorities say that clade I is considered a more aggressive form of mpox than clade II, which spread across the world and the United States in 2022 and 2023. A more recent outbreak in Africa centered in the Democratic Republic of Congo has been driven by the clade I strain, prompting declarations of emergencies across the continent earlier this year.

Between Jan. 1 and Nov. 15 of this year, about 12,000 mpox cases have been confirmed in Eastern and Central Africa, leading to at least 47 deaths, according to the CDC. Countries with large numbers of cases include the Democratic Republic of Congo, the Republic of the Congo, Uganda, Rwanda, Burundi, and the Central African Republic.

Other countries that have reported cases of the clade I strain include Germany, Sweden, the UK, India, Thailand, Kenya, and Zimbabwe.

“Data from affected countries indicate that a large proportion of clade I mpox cases among adults were associated with heterosexual contact,” the agency said. “Transmission to close contacts within households, including to children, also has been reported.”

The CDC also said local health officials across the United States at the state, local, and tribal level should report mpox cases “as soon as possible” to federal authorities, including clade II negative test results from any patients who recently traveled to a country that is currently impacted by clade I cases.

“Given the widespread outbreaks in Central and Eastern Africa, additional travel-associated cases may be reported in the future in the United States,” the CDC also said.

In confirming the case, the California Department of Public Health earlier this week said in a statement that the risk posed by the clade I mpox in the state is “very low,” adding there is “no concern or evidence” to show it is being transmitted between people in California or the United States.

But it said that the person who contracted clade I had returned from eastern Africa to Northern California. The patient is now isolating at home in San Mateo County, located south of San Francisco, according to the state health agency.

The CDC health alert called on local doctors, health care professionals, and state health officials to “consider mpox as a possible diagnosis in patients with epidemiologic characteristics and lesions or other clinical signs and symptoms consistent with mpox,” noting that they should take into account people who recently traveled to Central and Eastern Africa.

“This also includes people who had close or intimate contact with symptomatic people who have been in these countries,” the alert said.

The notice also urged people suspected of having the viral illness to stay home and isolate themselves until mpox has been ruled out via laboratory testing.

Symptoms of the disease can include fever, chills, and body aches, while more serious signs of the virus can include lesions on the face, hands, chest, and genitals, say health officials.

Over the summer, the World Health Organization (WHO) declared clade I mpox a “public health emergency of international concern,” it’s highest alert for a communicable disease. WHO Director-General Dr. Tedros Adhanom Ghebreyesus said at the time that a “coordinated international response is needed to stop these outbreaks and save lives.”