Thousands of people in northwest China’s Lanzhou city have tested positive for a bacterial infection, CNN reported on Thursday. The outbreak was caused by a leak at a state-owned biopharmaceutical company making animal vaccines last year.

The disease brucellosis, which is often caused by contact with livestock carrying the bacteria brucella, can cause symptoms including headaches, muscle pain, fever and fatigue. It is also known as the Malta or Mediterranean fever. However, human-to-human transmission of the virus is very rare, according to the United States’ Centers for Disease Control and Prevention. Most people are infected by eating contaminated food or inhaling the bacteria.

In a press release, the Health Commission of Lanzhou in Gansu province, confirmed that 3,245 people had contracted the disease. Another 1,401 people have tested as “preliminarily positive”. No fatalities have been reported so far due to the disease.

The authorities said they have tested 21,847 people out of the city’s 2.9 million (29 lakh) population. The Lanzhou Health Commission also announced on Tuesday that 11 public hospitals would provide free and regular checkups for the infected patients.

The health commission said the outbreak stemmed from a leak at the Zhongmu Lanzhou biological pharmaceutical factory, which occurred between late July to August last year. The factory while producing Brucella vaccines for animals, ended up using expired disinfectants and sanitisers. The bacteria was not eradicated in its factory exhaust and contaminated waste gas formed aerosols that contained the bacteria and leaked into the air.

This contaminated gas was carried by wind to the Lanzhou Veterinary Research Institute, where the outbreak of the disease was first reported. The first brucella infection at the institute was reported in November. By the end of December, at least 181 other people had been infected with brucellosis.

Infections were also reported from Heilongjiang province, at the very northeastern tip of the country, where 13 people, who had worked in the veterinarian institute in August, tested positive. Following this, the municipal and provincial authorities launched an investigation into the leak at the factory.

By January, they had revoked vaccine production licenses for the plant, and withdrew product approval numbers for its two Brucellosis vaccines. In February, the factory issued a public apology, and said it had “severely punished” eight people who were determined as responsible for the incident.