The World Health Organization separately announced on Friday that the global total had risen to 80 confirmed cases, with an additional 50 pending confirmation.

There are no more probable cases at this time, though several individuals may have the virus, including a person in New York City whose story has been circulating in the media, CDC epidemiologist Capt. Agam Rao told Fortune via email.

When asked how many individuals are “under interest” in the U.S. and in which states they live, the agency late Friday would only say that the “number fluctuates.”

Health officials are monitoring six people in the U.S., all of whom sat next to a person on a May 3-4 flight from Nigeria to London who eventually developed the virus, the CDC told Fortune on Wednesday in an emailed statement.

The global monkeypox cases are “atypical and, in fact, unprecedented,” Eric Toner, senior scientist and health security expert with the Johns Hopkins Bloomberg School of Public Health, told Fortune on Friday.

Such extensive person-to-person spread is not normal for the virus, he said. The pathogen is typically transmitted from human to human through close contact or large respiratory particles and is difficult to catch, with household transmission rates around 10% or lower.

Scientists have still not, however, determined how the recent cases are being transmitted, as a virus usually limited to Africa spreads around the globe, with cases reported in the UK, Spain, Portugal, Australia, Belgium, Canada, France, Germany, Italy, and Sweden.

“Until recently, most monkeypox cases in countries where the virus is not normally found had a direct link to ongoing monkeypox activity in Nigeria via travelers who visited Nigeria and returned to their home country,” Rao said. “The recent clusters in other countries are a new phenomenon that we are working to understand better.”

The pattern of transmission is highly unusual for the smallpox-related pathogen, which usually occurs in rural African areas where people have close contact with infected rats and squirrels, the Associated Press reported Friday, citing a Nigerian virologist who sits on several WHO advisory boards.

“This is not the kind of spread we’ve seen in West Africa, so there may be something new happening in the West,” he told the AP.

Monkeypox strains recently identified outside of Africa are of the West African clade, known to cause less severe illness than the other clade of monkeypox, the Congo Basin clade, Rao said.

“Sequencing work is ongoing, but based on what we know so far, it’s unlikely that this strain is more transmissible,” she told Fortune.

Experts have recently questioned whether the virus, deadly in about 10% of cases, is now being sexually transmitted, since a considerable number of cases in Europe have been identified among men who identify as having sex with men.

Monkeypox is not thought to be sexually transmitted, though it’s hard to tease out sexual contact from close contact, Andrea McCollum, an epidemiologist with the CDC’s Poxvirus and Rabies Branch, told Fortune on Wednesday.

“There is really no data on [monkeypox] presence in semen and vaginal fluids—the sexual transmission angle is really hard to tease apart” from the close contact that occurs during sex, she said.

Airborne transmission of monkeypox is “theoretically possible,” according to the CDC. Toner said he hasn’t seen evidence that suggests such transmission, “but we will continue to learn over the next several days.”

Airborne precautions for medical providers are recommended “out of an abundance of caution,” Rao said Friday, adding, “There is no reason to suspect airborne transmission.”

Monkeypox is a rare disease related to smallpox and cowpox, first identified in 1958 among colonies of monkeys kept for research, according to the CDC. Typically found in Africa, the virus causes fever, muscle aches, and lesions that progress through various stages before scabbing. The incubation period, or time between exposure and the onset of symptoms, is usually a week or two but can range from five to 21 days.