The U.S. Centers for Disease Control and Prevention (CDC) issued a health alert on Thursday to notify clinicians and health departments about a deadly type of the mpox virus spreading in the Democratic Republic of the Congo (DRC).

The CDC said it was alerting about the possibility of a subtype of the mpox virus called Clade I in travelers who have been in DRC.

Clade I is generally more infectious and leads to more severe infections than another subtype called Clade II. Clade I has not yet been reported in the United States at this time, the CDC added.

Recent evidence has shown for the first time Clade I can be transmitted through sexual contact, which has proved to be the main transmission mode for the less deadly strain of the disease that broke out globally last year, chiefly among gay, bisexual and other men who have sex with men.

It was previously thought that both types of mpox, which causes flu-like symptoms and pus-filled skin lesions, spread mainly through close contact among humans after spilling over from an infected animal through bites, scratches, hunting or cooking.