Health authorities in North America and Europe have detected dozens of suspected or confirmed cases of monkeypox since early May, sparking concern the disease endemic in parts of Africa is spreading.
Canada was the latest country to report it was investigating more than a dozen suspected cases of monkeypox, after Spain and Portugal detected more than 40 possible and verified cases.
Britain has confirmed nine cases since 6 May and the United States verified its first yesterday.
The US health authorities said a man in the eastern state of Massachusetts had tested positive for the virus after visiting Canada.
The illness, from which most people recover within several weeks and has only been fatal in rare cases, has infected thousands of people in parts of Central and Western Africa in recent years but is rare in Europe and North Africa.
The World Health Organization said on Tuesday it was coordinating with UK and European health officials over the new outbreaks.
Speaking this morning on RTÉ's Today with Claire Byrne, WHO infectious disease epidemiologist Dr Maria Van Kerkhove said they are concerned about the virus and stated there needs to be a greater focus on understanding transmission patterns.
She said "very little attention" has been paid to monkeypox as it is not something that is perceived as a global risk, but that the WHO considers it to be a "priority pathogen".
Ms Van Kerkhove appealed to countries globally "to be ready" to be able to detect cases of monkeypox and offer appropriate care to prevent any onward spread.
She said they are concerned about the spread to non-endemic areas who do not have the virus "on their radar" after a number of cases were reported in European countries and the US without travel links.
"So we are trying to expand our understanding of the circulation of this virus which typically is transmitted between people through contact, physical contact with lesions, with open sores.
"And so we are working with a number of countries to identify who is infected and of course to prevent onward transmission and they receive the appropriate care," she said.
The first case in Britain was someone who had traveled from Nigeria, though later cases were possibly through community transmission, the UK Health Security Agency (UKHSA) said in a statement.
"These latest cases, together with reports of cases in countries across Europe, confirms our initial concerns that there could be spread of monkeypox within our communities," said UKHSA Chief Medical Adviser Dr Susan Hopkins.
The WHO said it was also investigating that many cases reported were people identifying as gay, bisexual or men who have sex with men.
"We are seeing transmission among men having sex with men," said WHO Assistant Director-General Dr Soce Fall at a press conference earlier this week.
"This is new information we need to investigate properly to understand better the dynamic of local transmission in the UK and some other countries."
'No risk to the public'
The UKHSA noted that monkeypox has not previously been characterised as a sexually transmitted disease, underscoring that "it can be passed on by direct contact during sex".
"Anyone, regardless of sexual orientation, can spread monkeypox through contact with body fluids, monkeypox sores, or shared items (such as clothing and bedding) that have been contaminated with fluids or sores of a person with monkeypox," a US Centers for Disease Control and Prevention (CDC) statement said, adding that household disinfectants can kill the virus on surfaces.
The illness often starts with flu-like symptoms such as fever, muscle ache and swollen lymph nodes before causing a chickenpox-like rash on the face and body, the US agency explained.
The Massachusetts Department of Health, said that the case there - the first confirmed this year in the US - occurred in a patient who had recently travelled to Canada and "poses no risk to the public, and the individual is hospitalised and in good condition".
Health authorities in Canada's Quebec province announced they were investigating at least 13 suspected cases of monkeypox, the public broadcaster CBC reported yesterday.
The cases were flagged to Montreal authorities after diagnoses were made in several clinics specialising in sexually transmitted and blood-borne infections.
The Public Health Agency of Canada (PHAC) told CBC it had called on "public health authorities and laboratory partners across Canada to be alert for and investigate any potential cases".
According to the CDC, there were no reported cases of monkeypox for 40 years before it re-emerged in Nigeria in 2017.