Health authorities confirmed Wednesday that monkeypox—an infectious disease that’s usually found in parts of Africa—has arrived in the U.S. after several cases were reported across Europe.

The first case associated with the current outbreak was reported in the U.K. on May 7 in a person who had recently traveled to Nigeria, where they are believed to have contracted the disease.

Five cases of monkeypox have since been confirmed in Portugal, while Spain has seven cases.

Canada is investigating 13 suspected cases of the disease, with the U.S. patient reported to have recently traveled to the country.

What is monkeypox, and where did it come from?

Monkeypox is a rare zoonotic disease—meaning it’s caused by germs that spread between animals and people.

It’s a virus that is mainly spread by wild animals in parts of west and central Africa, according to the WHO—but human-to-human transmission can also occur, and it is occasionally exported to other parts of the world when an infected person travels.

There are two variants of the virus: the West African strain and the Central African strain.

The cases identified in the U.K. have all been the West African strain of the virus, which is thought to be milder than the Central African strain.

What are the symptoms?

According to Britain’s National Health Service, the first symptoms of monkeypox include fever, headaches, muscle aches, exhaustion, and swollen glands.

A rash appears one to five days after the first symptoms, often beginning on the face before spreading to other parts of the body.

The rash, which starts as raised spots that turn into blisters before falling off, is often confused with chicken pox, the NHS says.

In countries like the U.K. or the U.S., symptoms are typically mild, Professor Neil Mabbott, personal chair of immunopathology at the University of Edinburgh’s Roslin Institute, told Fortune in a phone call on Thursday.

However, he said there is currently not enough data to determine whether certain demographics are at heightened risk of developing more severe symptoms.

How do you catch the disease?

The disease is usually caught from an infected animal if a person is bitten or comes into contact with its blood or other bodily fluids, or they consume or touch infected animal products like meat, skin, or fur.

While human-to-human transmission is rare, it is possible.

The U.K. Health Security Agency (UKHSA) has identified nine cases of monkeypox in Britain since the country first detected a single case of the virus on May 7.

Two cases of the virus were identified in London a week after the first case was reported, but authorities said the individuals had no connection to the previously confirmed case, and “where and how they acquired their infection remains under investigation.”

The UKHSA detected four additional cases on May 16, and a further two on May 18.

Common contacts were only identified for less than half of the new infections.

As the latest two patients had no travel links to a country where monkeypox is endemic, the UKHSA said it was possible that they had contracted the disease through community transmission.

Despite this possibility, health authorities have emphasized that the risk to the general public remains low.

“As a whole, we shouldn’t be panicking—this is not a disease that spreads like COVID-19 within the general population,” Mabbott told Fortune. “Monkeypox isn’t very transmissible in humans. It needs very, very close contact with an infected person to be able to transmit.”

He explained that the virus transmits via large droplets that can only travel a short distance, but it can also be spread through direct contact with skin lesions or, sometimes, contaminated bedsheets or clothes.

Andrew Preston, a professor of microbial pathogenicity at the University of Bath, also told Fortune on Thursday that human-to-human spread is thought to be relatively rare and difficult.

“So these current cases that we’re discussing are somewhat surprising,” he said.

“There have been cases of people [picking it up from] contaminated bedding or fluid from the skin of an infected person, so there are various different opportunities to get the virus, but it’s [generally through] pretty close contact, we think, with an infected person.”

Preston said it was still unclear whether the amount of time spent with an infected person made someone more or less likely to catch the virus.

“It’s still somewhat of a mystery as to how this is spreading. The chances are it’s just not being recognized—it’s not the first thing you think of, and some people say the rash is a bit like shingles,” he said.

He added that it was concerning to see that some of the cases identified in the U.K. had no links to one another.

“That would suggest there are other contacts forming the connections between those cases,” he explained. “That’s the worrying thing: The epidemiology would suggest there are other unrecognized cases at the moment. There have to be other cases in the community.”

Is monkeypox sexually transmitted?

The U.K.’s most recent cases of monkeypox were predominantly found in gay or bisexual men, or men who have sex with men, according to health officials.

Monkeypox has not previously been described as a sexually transmitted infection, but British health officials said on Wednesday that the virus can be passed by direct contact during sex.

“We are particularly urging men who are gay and bisexual to be aware of any unusual rashes or lesions and to contact a sexual health service without delay if they have concerns,” Dr Susan Hopkins, the UKHSA’s chief medical adviser, said in a statement on Wednesday.

However, according to Mabbott, any transmission of monkeypox between sexual partners is likely to be down to close contact, rather than the virus spreading in the same way as other sexually transmitted infections.

“We can’t rule that out, but it is most likely due to the very close contact,” he said.

Preston added: “Monkeypox is not recognized as a sexually transmitted disease—that is, something that’s hugely dependent on being spread through sexual contact. There’s nothing to suggest that here, but if you’ve got a rash on your skin that’s potentially infectious, or you’re breathing contaminated droplets, then if you’re up close with someone [it can spread]. That doesn’t necessarily mean it is a sexually transmitted disease like gonorrhea or chlamydia.”

Can you die from monkeypox?

Fatality rates differ between the two strains, Mabbott said on Thursday.

The West African variety, the strain currently circulating in Europe and the U.S., has a recorded fatality rate of around 1%, while the Central African strain’s recorded fatality rate is around 10%.

“It can be fatal, and it’s most likely to be fatal somewhere where there is [poor] health care,” Mabbott told Fortune.

Is there a cure?

Mabbott noted that while there was currently no cure for monkeypox, smallpox vaccines have been used in Africa in the past to provide protection against monkeypox, as the viruses are related.

However, a successful widespread vaccination campaign eradicated smallpox from circulation decades ago, meaning the majority of the population hasn’t been given a vaccine for the virus.

“Most of the world hasn’t been vaccinated against smallpox, so most people would be completely susceptible to [monkeypox],” Preston told Fortune.

“We haven’t been using that vaccine for about 40, 50 years, so there are very few people walking around now that have smallpox-vaccine-induced immunity,” Mabbott agreed.

“Likewise, I’m not aware of any antiviral treatments which have been approved for use against monkeypox. So we don’t have those tools.”

This story was originally featured on Fortune.com