Vaccinia virus (VACV or VV) is a large, complex, enveloped virus belonging to the poxvirus family. It is well-known for its role in eradicating smallpox. It was the first human disease to to successfully be eradicated by science. The World Health Organization was the one who completed the endeavor.
The vaccinia virus saw renewed interest in the early 21st century due to concerns about the smallpox virus being used as an agent for bioterrorism. They can be divided into four types: Generalized vaccinia, Eczema Vaccinatum, Progressive vaccinia, Roseloa vaccinia.
It is closely related to the virus that causes Cowpox; historically the two were often considered to be one and the same. Vaccinia virus comes from an unknown origin but it is commonly believed to be derived form an ancestral virus of cowpox and Variola virus.
Poxviruses are unique because they replicate in the cytoplasm of the host cell, outside of the nucleus. Infection is typically mild and asymptomatic in healthy individuals. It can cause mild rash and fever. The immune system protects individuals from a lethal smallpox infection. The vaccine doesn’t actually contain any smallpox so it cannot cause the infection although it can create other problems.