An explosion started a subsequent fire at the Vector lab in Russia, which is known for being one of two places in the world to house the smallpox virus.
One man is being treated in intensive care after an explosion and subsequent fire ripped through a lethal viruses laboratory in Russia on Monday.
The Vector laboratory, situated in the Novosibirsk region, said in a statement that a gas cylinder exploded during repair works of a sanitary inspection room on the fifth floor of the six-storey building.
No work with biological material was being carried out at the time, it said, and the building structure was not damaged by the fire.
TASS news agency reported that the injured person worked for a construction company, and is currently being treated in hospital for burns.
According to the National Institute of Allergy and Infectious Diseases (NIH) in the US, the laboratory is one of two places on Earth that houses the smallpox virus. The other is in Atlanta, Georgia.
Smallpox is described by the World Health Organisation (WHO) as "one of the world's most devastating diseases known to humanity", with the last case being reported in Somalia in 1977.
It has been declared eradicated since 1980.