“A lot of people are working hard around the clock, supporting state health departments—not just in this case, but in preparation for other cases that may occur,” Andrea McCollum, an epidemiologist with the CDC’s Poxvirus and Rabies Branch, told Fortune late Wednesday.

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. It’s thought to be fatal in about 10% of cases.

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

Additional cases have recently been reported in the U.K., Britain, Portugal, and Spain. British health officials are concerned that “multiple chains” of transmission may be occurring in the country, given the lack of travel history and the geographic dispersion of the country’s emerging patients.

“What we’re seeing in Europe is a pretty substantial localized transmission among certain communities,” McCollum says, adding that the U.S., so far, has not seen the same phenomenon, with no additional suspected or confirmed cases as of Wednesday evening.

“Here, we have one single case, and the state health department is working hard to run down potential contacts and potentially offer a vaccine,” McCollum says, referring to the smallpox vaccine, used to prevent infections of monkeypox, which is also a orthopoxvirus.

A new STD?

Some European officials worry that the continent is seeing a new mode of spread due to atypical diseases clusters, the Associated Press reported Wednesday, as many identified cases have been among men who have sex with other men. But data to support the theory is currently lacking, McCollum says.

“There is really no data on [monkeypox] presence in semen and vaginal fluids—the sexual transmission angle is really hard to tease apart,” McCollum says. Because respiratory droplets and close contact are known modes of transmission, McCollum adds, it’s hard to differentiate potential sexual transmission from close-contact transmission that may occur during sex.

Many global case reports “are occurring within sexual networks,” said Dr. Inger Damon, a poxvirus expert and director of the CDC’s Division of High-Consequence Pathogens and Pathology, in a Wednesday news release from the agency.

“However, healthcare providers should be alert to any rash that has features typical of monkeypox. We’re asking the public to contact their healthcare provider if they have a new rash and are concerned about monkeypox,” Damon said.

Some recent monkeypox patients have had genital lesions that “may be hard to distinguish from syphilis, herpes simplex virus infection, chancroid,” and other more common STDs, according to the CDC.

While sexual transmission of monkeypox has never been documented, the disease has experienced “epidemiological changes in recent years,” including changes in the age of those affected and location of infections, according to a Wedneday health security update sent by the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health.

‘Less severe than smallpox ever was’

While concerning, monkeypox is “not highly transmissible” and “not likely to cause large outbreaks,” says Dr. Eric Toner, a senior scientist and health security expert with the Johns Hopkins Bloomberg School of Public Health.

There’s a likelihood that more cases will be identified in the U.S., but “we know how to contain it, vaccinate against it,” Toner said. “It has a long incubation period, which means plenty of time to implement quarantine and isolation.”

Those at highest risk are household contacts of those infected, and “even amongst those, at least what we know from the outbreaks in Africa, the attack rate within a household is under 10%. It’s not highly transmissible within households,” Toner said.

Monkeypox is milder than smallpox, and COVID-19 prevention measures like masking will inhibit transmission, said Dr. Alexandra Brugler Yonts, an infectious disease specialist at Children’s National in Washington, D.C., who assisted in the FDA’s review of smallpox vaccine Jynneos, also used to treat monkeypox.

“Monkeypox is thankfully less severe than smallpox ever was, with a lower mortality rate,” she said.

And there’s more good news: Common household disinfectants can kill the monkeypox virus on surfaces, according to the CDC.

Vaccines galore in the U.S. Strategic National Stockpile

A number of treatments for monkeypox exist, even if none are specific to the virus.

Smallpox vaccine Jynneos is approved to treat monkeypox in adults because of evidence that animal-transmitted pox viruses like monkeypox and rabbitpox “typically cross-react and provide protection against other pox viruses,” Brugler Yonts said. Other smallpox vaccines were used to prevent monkeypox transmission in outbreaks before the development of Jynneos, though they weren’t approved for such a purpose, she added.

Antivirals are available too, she said, including tecovirimat, also known as TPOXX, an oral antiviral approved for smallpox in adults and children. In the European Union it’s approved for the treatment of monkeypox and cowpox “and would likely be allowed to be given off label in the case of a monkeypox case in the U.S.,” Yonts said.

There’s also cidofovir, an antiviral used for other viruses, like adenovirus, with demonstrated success against monkeypox that could also be used in humans, she added.

Such vaccines and antivirals are included in the Strategic National Stockpile in the U.S., Yonts said. And as of 2001 there were enough smallpox vaccines in the stockpile for the entire U.S. population, according to Toner.

In short, says Toner: “It is of no risk to the general public, and there are plenty of other things to worry about.”