Mpox (formerly Monkeypox)

image of mpox virus

Colorized scanning electron micrograph of mpox virus (orange) on the surface of infected VERO E6 cells (green).

Credit: NIAID

The monkeypox virus (mpxv) is part of the Orthopoxvirus genus, which also includes variola virus (the cause of smallpox), vaccinia virus, and cowpox virus. It causes mpox disease, formerly known as monkeypox disease. Although mpox is similar to smallpox, it is much less deadly. Initial symptoms of mpox include: fever, headache and body aches, fatigue, and swollen lymph nodes, followed by a rash of lesions on the skin. Human-to-human transmission of mpxv occurs through direct contact with body fluids, lesions, prolonged face-to-face contact, including sexual contact, and indirect contact with contaminated clothing or bedding. 

The virus was first discovered in 1958 in two outbreaks of a pox-like disease among colonies of research monkeys. The first human case of mpox was recorded in 1970 in the Democratic Republic of the Congo (DRC). The disease is endemic in central and western Africa.

2022 Outbreak

In 2022, an outbreak of cases began occurring in multiple countries where mpox had not normally occurred, including the United States. Gay, bisexual, and other men who have sex with men comprise a high number cases; however, anyone who has been in close contact with someone who has mpox is at risk for infection.

Mpxv virus variants are referred to as clades: Clade I (formerly known as the Congo Basin or Central African clade) and Clade II (formerly known as the West African clade). Clade II is further divided into subclades: Clade IIa and Clade IIb. According to the World Health Organization, cases identified as part of the ongoing global outbreak are largely caused by Clade IIb. Clade I is responsible for infections in the DRC and is estimated to cause more severe disease and higher mortality than Clades IIa and IIb.


Currently, there is no specific treatment approved for mpox. However, there are antiviral medications that can be used to treat smallpox and other conditions that may help patients with mpox. NIAID supported the development of tecovirimat (TPOXX) and is evaluating the drug in two clinical trials for mpox. 

Read more about mpox treatment


NIAID provided significant support in the development of the JYNNEOS™ (also known as Imvamune or Imvanex) vaccine to prevent mpox and smallpox. The vaccine was developed for people with weakened immune systems who were at risk for severe side effects from the existing smallpox vaccines. NIAID is evaluating alternative dosing approaches for the JYNNEOS vaccine in a clinical trial. 

Read more about mpox vaccines
Content last reviewed on