More than 300 cases of monkeypox have been reported across at least 20 countries, most of which are in Europe — a rare occurrence for a virus largely confined to central and western Africa.

The U.S. has identified nine cases across seven states as of Thursday. Fourteen European countries have reported cases, with the highest totals in the U.K., which has confirmed 85 infections, and Spain at 84.

Dr. David Heymann, a leading adviser to the World Health Organization, said last week that two raves in Spain and Belgium may have fueled transmission. Public health authorities in Madrid have also linked Spain's infections to an outbreak at a sauna.

Outside of Europe and the U.S., cases have been reported in Australia, Canada, Israel and the United Arab Emirates. Several countries have more suspected cases awaiting confirmation. No deaths have been reported.

"This is the most important outbreak in the history of monkeypox in the Western Hemisphere," said Anne Rimoin, an epidemiology professor at UCLA Fielding School of Public Health.

The last time the Western Hemisphere saw a monkeypox outbreak of this magnitude was in 2003, she said, when the U.S. identified 47 cases. Those patients had been in contact with infected pet prairie dogs, and none died.

"What we're facing right now seems to be at least a subset of cases that don’t have any history of travel to one of those countries in Africa where the monkeypox virus naturally occurs, and also don’t report any exposure to someone who has been diagnosed with monkeypox. So what we’re seeing right now is unusual," said Dr. Agam Rao, a medical officer at the Centers for Disease Control and Prevention's Division of High Consequence Pathogens and Pathology, said last week.

Because some of the U.S. patients do not have a history of travel to areas where monkeypox has been spreading recently, CDC Director Rochelle Walensky said on Thursday that "we need to presume that there is some community spread."

Monkeypox belongs to the family of poxviruses, which includes smallpox. The disease got its name after scientists discovered it among laboratory monkeys in 1958. The first monkeypox case in a human was diagnosed in 1970.

Since then, most infections have been concentrated in the Democratic Republic of the Congo and Nigeria. The DRC reports thousands of cases annually and Nigeria has reported more than 200 confirmed cases and more than 500 suspected ones since 2017.

The type of monkeypox identified in the recent U.S. and European cases tends to produce milder disease than the other common branch of the virus.

"All of the virus strains that we're aware of among all of these cases that have occurred in the last two weeks are the West African clade. The West African clade of monkeypox is much more benign than the Congo Basin clade," Rao said. "That’s good news in that, hopefully, there will not be a lot of bad things clinically that happen to people who might be infected."

Around 1 percent of people who contracted the West African clade in the past have died, compared to up to 10 percent of people who contract the Congo Basin clade, according to the World Health Organization.

Rao said people who get the West African clade "typically recover pretty well" and go "back to their regular lives when it’s over."

Humans can get monkeypox from animals, either through bites or scratches or preparing meat from wild game, according to the CDC.

Person-to-person transmission can occur through the exchange of large respiratory droplets during prolonged face-to-face contact. People can also get exposed through direct contact with bodily fluids, the lesions that form during an infection, or contaminated items like clothing or bedding.

Many of the newly identified cases are among men who have sex with men, but monkeypox is not considered a sexually transmitted infection.

"It’s probably premature and potentially even harmful to assume that there are only cases within that community," Rao said.

She added that the overrepresentation of this group may simply be a product of skin-to-skin contact within a tight-knit community.

"There's going to need to be studies related to trying to isolate virus from seminal fluid or vaginal fluid. There’s really quite a lot of work that would need to be done before we would say that it can be transmitted sexually," she said.

WHO officials said Monday that monkeypox might be spreading through exposure to rashes and lesions during sexual activity.

"Many diseases can be spread through sexual contact. You can get a cough or a cold through sexual contact, but it doesn't mean that it's a sexually transmitted disease," said Andy Seale, an advisor with the WHO's HIV, Hepatitis and STIs Program.

Monkeypox symptoms can develop five to 21 days after someone is infected. Most people recover after two to four weeks.

The illness typically starts with flu-like symptoms such as fever, headache, muscle aches and exhaustion, which can last a day or two. A rash often follows one to three days after the fever, progressing from red areas to small bumps on the skin. Those can then turn into blisters that may fill with whitish fluid.

The rash sometimes looks similar to chickenpox, syphilis or herpes. It typically spreads from the face to the limbs, hands, feet and then to the rest of the body, WHO officials said. But disease experts have identified a pattern among recent cases.

"We're seeing more cases where the rash begins in the genital area — which is not new, that has always been there — but it's more frequent now and sometimes it tends to stay there," said Dr. Rosamund Lewis, Head of the Smallpox Secretariat at the WHO.

That could make the rash less visible, she added.

Following the identification of the first U.S. case, the CDC instructed health care providers to look out for patients with the characteristic monkeypox rash.

"We are recommending that all clinicians do this, but in particular, those caring for patients in STD clinics," Rao said, referring to sexually transmitted diseases.

So far, Rimoin said, the recent infections "appear to be reasonably mild cases that have been found through clinics, not because people are presenting severely ill to the emergency room."

Rimoin said it makes sense that new cases of monkeypox continue to crop up, since there’s less immunity to poxviruses than there was before 1980, when people still received smallpox vaccines.

"It’s not surprising that we’re seeing infections as a result of exposures, given that we no longer have that immunity that we counted on during the eradication of smallpox era," she said.

There is not a proven treatment for monkeypox, but doctors can treat its symptoms, and two antivirals for smallpox could also be considered. Rimoin said supportive care is pretty effective for the West African clade.

Doctors who identify a suspected monkeypox case should report it to the CDC, Rao said, since "any potential treatments that might be provided to the patient are really only available through consultation with public health authorities."

Older smallpox vaccines are around 85 percent effective at preventing monkeypox, but the U.S. stopped vaccinating the public for smallpox in 1972. In 2019, the Food and Drug Administration approved a smallpox vaccine that also protects people from monkeypox. Experts think that vaccine could help reduce symptoms or prevent disease if administered shortly after someone gets infected.

The CDC website states that "in the event of another outbreak of monkeypox in the U.S., CDC will establish guidelines explaining who should be vaccinated."

For now, Rao said, the risk to the general population is low.

"I would not want people to be too terribly alarmed right now and to be changing their behaviors too much," she said.

Aria Bendix is the breaking health reporter for NBC News Digital.