GUANGZHOU -- Four people in South China's Guangdong Province who have recently returned from south Asia have contracted malaria, said an official with the provincial Inspection and Quarantine Bureau (IQB).

The four workers, who all belong to an enterprise in Shunde City whose name the official declined to reveal, are in hospital. One of them is in critical condition.

They worked in south Asia in April and May and, since getting home in September, have been suffering from fever, diarrhea, skin rashes and dizziness.

All of them were bitten by mosquitoes.

Malaria is one of the world's most common diseases, caused by a parasite that is transmitted to humans by a female mosquito's bite. Malaria symptoms include fever, shivering, pain in the joints, headache, repeated vomiting, generalized convulsions and coma. If not treated, the most serious types of malaria can prove fatal in as little as two days. Malaria parasites weaken the immune system, making people more vulnerable to other infectious, life-threatening diseases.

The workers are quarantined and the IQB has launched a campaign to exterminate mosquitoes in the province.

Guangdong has also reported 492 dengue cases, with no deaths.

Infectious diseases accounted for 59 percent of the 11,109 public health emergencies in September, according to an earlier report from the State Health Ministry.