Dr Amesh Adalja, from John Hopkins University, said patients were infectious for as long as they had symptoms including skin lesions. Virus can be passed on by touch or via the air.
Many cases have been detected in gay and bisexual men, with scientists saying it is possible the virus was passed on sexually.
In Spain — which has detected 30 cases, the most out of any nation — infections are being linked to a single un-named 'sauna' and are among young gay and bisexual men. In the country, sauna is slang for a place where men who have sex with other men meet for sex, rather than a bathhouse.
What is monkeypox?
Monkeypox is a rare viral infection which people usually pick up in the tropical areas of west and central Africa.
It is usually spread through direct contact with animals such as squirrels, which are known to harbour the virus.
However, it can also be transmitted through very close contact with an infected person.
Monkeypox was first discovered when an outbreak of a pox-like disease occurred in monkeys kept for research in 1958.
The first human case was recorded in 1970 in the Democratic Republic of Congo and the infection has been reported in a number of central and western African countries since then.
Only a handful of cases have been reported outside of Africa until now and they were confined to people with travel links to the continent.
How deadly is it?
Monkeypox is usually mild, with most patients recovering within a few weeks without treatment. Yet, the disease can prove fatal.
However it can kill up to 10 per cent of people it infects.
The milder strain causing the current outbreak kills one in 100 — similar to when Covid first hit.