But it is not purely a sexually transmitted infection (STI), and can also be passed on by close contact with an infected person.
The virus typically causes pimple-like rashes, which can be extremely itchy and painful, to develop and spread across the body, as well as other complications.
Infections are usually mild and clear up on their own in adult patients without treatment or hospitalisation – but the World Health Organization (WHO) has warned that young children have died at higher rates from the disease.
Officials in the US and elsewhere have recommended that people at the highest risk of exposure – including some gay and bisexual men, as well as some healthcare workers – should get the jab on a priority basis.
The declaration on Thursday comes amid reports that vaccines and treatments for monkeypox are in short supply across the country.
The Department of Health and Human Services said last week that it has ordered more than five million more doses, to be delivered until next May.
Earlier this week, the White House also announced the appointment of a team to co-ordinate and oversee the national monkeypox response. It includes Dr Demetre Daskalakis, a New York physician who has led successful initiatives to curb the spread of HIV in the US.
Source: BBC News