New outbreak disease, also known as Malta fever or Mediterranean fever, can cause symptoms including headaches, muscle pain, fever and fatigue. While these may subside, some symptoms can become chronic or never go away, like arthritis or swelling in certain organs, according to the United States' Centers for Disease Control and Prevention (CDC).
Human-to-human transmission is extremely rare, according to the CDC. Instead, most people are infected by eating contaminated food or breathing in the bacteria - which seems to be the case in Lanzhou.
This outbreak stemmed from a leak at the Zhongmu Lanzhou biological pharmaceutical factory, which occurred between late July to late August last year, according to the city's Health Commission. While producing Brucella vaccines for animal use, the factory used expired disinfectants and sanitisers - meaning not all bacteria were eradicated in the waste gas.
The bacteria brucella is carried by cattle but can infect humans. (AAP) This contaminated waste gas formed aerosols that contained the bacteria - and leaked into the air, carried by wind down to the Lanzhou Veterinary Research Institute, where the outbreak first hit.
People at the institute began reporting infections in November, and it quickly accelerated. By the end of December, at least 181 people at the institute had been infected with brucellosis, according to Chinese state-run news agency Xinhua.
