The pathogen enters the body through damaged skin, the respiratory tract or the mucous membranes of the eyes, nose, mouth, rectum and anus.
The main animal source of the disease has not yet been identified, but infected rodents are suspected of playing an important role in transmission in parts of Africa.
During the first major outbreak in 2022, the virus spread mainly among homosexual men.
The new variant appears to be more contagious and is reportedly infecting many people without sexual contact. The emergence of a new mutated strain, or clade, is concerning because it indicates the virus is adapting and getting better at spreading from person to person.
What is a global public health emergency and why was it declared?
A “public health emergency of international concern” (PHEIC) is the WHO's highest level of alert.
They are declared when diseases spread in new or unusual ways and are intended to stimulate international cooperation and funding to combat an outbreak.
The WHO statement follows a similar statement from the Africa Centres for Disease Control and Prevention earlier this week.
WHO chief Tedros Adhanom Ghebreyesus said after declaring the emergency that the potential for further spread within Africa and beyond is “very worrying”.
“A coordinated international response is essential to stop this outbreak and save lives,” he said.