Monkeypox, often transmitted by rats and other rodents, kills around one in 10 of its victims and can be particularly dangerous in children.
The disease is not normally found outside Africa, but Public Health England confirmed two people have contracted the disease for the first time ever in the UK.
The condition was diagnosed in a patient in Cornwall last week and just days later, a second case was detected at Blackpool Victoria Hospital.
Health officials have now warned the virus has a limited secondary spread through human-to-human transmission.
The virus enters the body through broken skin, the respiratory tract, or the mucous membranes.
Victims of the illness can suffer with a fever and rash, which can turn into plague like symptoms such as blisters and boils appearing on the body.
It can be transferred through animal bites and then contact with a victim including coughing and sneezing.
It was confirmed a man visiting from Nigeria was diagnosed with the rare viral infection in Cornwall on Friday.
He was taken to a specialist hospital in London after visiting a naval base in the region.
A PHE spokesman said the man had travelled on a commercial flight to the UK.
He subsequently travelled from London to Cornwall.
Officials have been assisted by the airline to identify passengers who sat near the man, who is believed to have contracted the infection in Nigeria before travelling to the UK.
The second patient to test positive had also recently travelled to Nigeria, where they are believed to have caught the virus.
But the two cases are thought to be unrelated.
In the Blackpool case, the person, who has not been named, returned home from the African country and fell ill.
They are currently being treated in a specialist unit at Liverpool University Hospital – an expert centre for infectious respiratory diseases.
Dr Nick Phin, deputy director of the National Infection Service at PHE, said: "We know that in September 2017 Nigeria experienced a large sustained outbreak of Monkeypox and since then sporadic cases have continued to be reported.
"It is likely that Monkeypox continues to circulate in Nigeria and could, therefore, affect travellers who are returning from this part of the world.
"However, it is very unusual to see two cases in such a relatively short space of time.
"We are working hard to contact individuals, including healthcare workers, that might have come into contact with the individual to provide information and health advice.”
The disease has been spread to 10 different countries in Africa in the last 40 years.
It has proved particularly devastating in the Congo Republic, which has had 88 cases of the disease, and six deaths.
Monkeypox outbreaks can be controlled through smallpox vaccines, but supplies have gone down since the disease was eradicated.
There are no other specific treatments available for the disease.
