Cases of monkeypox – a rare, little-known disease – are being investigated in European countries including the UK, the US, Canada and Australia.
On Friday the UK confirmed the number of cases had more than doubled, bringing the total to 20.
Health Secretary Sajid Javid said 11 new cases had been confirmed and most cases were mild.
The UK government has bought stocks of smallpox vaccine to guard against monkeypox.
How common is monkeypox?
Monkeypox is caused by the monkeypox virus, a member of the same family of viruses as smallpox, although it is much less severe and experts say chances of infection are low.
It occurs mostly in remote parts of central and west African countries, near tropical rainforests.
There are two main strains of virus – west African and central African.
Two of the infected patients in the UK travelled from Nigeria, so it is likely that they are suffering from the West African strain, which is generally mild, but this is as yet unconfirmed.
Another case was a healthcare worker who picked up the virus from one of the patients.
More recent cases do not have any known links with each other, or any history of travel. It appears they caught it in the UK from spread in the community.
The UK Health Security Agency says anyone with concerns that they could be infected should see a health professional, but make contact with the clinic or surgery ahead of a visit.
What are the symptoms?
Initial symptoms include fever, headaches, swellings, back pain, aching muscles and a general listlessness.
Once the fever breaks a rash can develop, often beginning on the face, then spreading to other parts of the body, most commonly the palms of the hands and soles of the feet.
The rash, which can be extremely itchy, changes and goes through different stages before finally forming a scab, which later falls off. The lesions can cause scarring.
The infection usually clears up on its own and lasts between 14 and 21 days.
How do you catch it?
Monkeypox can be spread when someone is in close contact with an infected person. The virus can enter the body through broken skin, the respiratory tract or through the eyes, nose or mouth.
It has not previously been described as a sexually transmitted infection, but it can be passed on by direct contact during sex.
It can also be spread by contact with infected animals such as monkeys, rats and squirrels, or by virus-contaminated objects, such as bedding and clothing.
How dangerous is it?
Most cases of the virus are mild, sometimes resembling chickenpox, and clear up on their own within a few weeks.
Monkeypox can sometimes be more severe, however, and has been reported to have caused deaths in west Africa.
How common are outbreaks?
The virus was first identified in a captive monkey and since 1970 there have been sporadic outbreaks reported across 10 African countries.
In 2003 there was an outbreak in the US, the first time it had been seen outside Africa. Patients caught the disease from close contact with prairie dogs that had been infected by small mammals imported into the country. A total of 81 cases were reported, but none resulted in deaths.
In 2017, Nigeria experienced the largest known outbreak. There were 172 suspected cases and 75% of victims were men between 21 and 40 years old.
What is the treatment?
There is no treatment for monkeypox, but outbreaks can be controlled by infection prevention.
Vaccination against smallpox has been proven to be 85% effective in preventing monkeypox.
The UK has bought doses of the smallpox vaccine, but it is not clear how many jabs might be given.
It has also been reported that Spain is preparing to order thousands of doses.
Should the public be concerned?
Experts say we are not on the brink of a national outbreak and, according to the UK Health Security Agency (UKHSA), the risk is low.
Prof Jonathan Ball, professor of molecular virology, University of Nottingham, said: “The fact that only one of the 50 contacts of the initial monkeypox-infected patient has been infected shows how poorly infectious the virus is.
“It is wrong to think that we are on the brink of a nationwide outbreak.”
UKHSA is following up those who have had close contact with patients to offer advice and to monitor them.