Chikungunya typically produces symptoms including fever, muscle pain, nausea, fatigue and a rash.
But in rare cases, it can cause debilitating joint pain that persists for months or even years. Patients who get severely ill often require hospitalization because of the risk of organ damage.
The World Health Organization says severe cases and deaths are rare and mostly occur in babies or elderly people with underlying health conditions.
There is no specific treatment for chikungunya, but health workers can treat the symptoms by giving medicines to lower fevers or ease muscle pain.
Two vaccines have been approved in several regions, including Britain, Brazil, Canada and Europe. Those are mostly targeted at travelers and are not widely available in the countries most affected by chikungunya.
Chikungunya causes regular outbreaks in Africa, Asia and the Americas, with occasional small epidemics in Europe.
As of July, there have been about 240,000 cases of chikungunya, including 90 deaths in 16 countries, according to the European Centres for Disease Prevention and Control. The countries that reported the highest number of infections were Brazil, Bolivia, Argentina and Peru.
With thousands of confirmed cases, this appears to be the biggest chikungunya outbreak ever documented in China, according to César López-Camacho of the University of Oxford.
“What makes this event notable is that chikungunya has never been established in mainland China before,” he said in a statement. “This suggests that most of the population had no preexisting immunity, making it easier for the virus to spread quickly.”
