Calcutta: A 40-year-old man died on Wednesday afternoon after complaining of diarrhoea since morning at Baghajatin, the suburb in the grip of an outbreak of the enteric disease.

Biswajit Das, a resident of North 24-Parganas' Ichhapore, had been staying in Baghajatin's J block since February 2 for a family wedding. Biswajit used to work in a sari shop in Ichhapore and his wife is pregnant, said a family member. The couple have twin daughters aged 8.

Over 3,500 people have been affected by the outbreak since Friday evening. The affected areas are spread over nine CMC wards and include places like Chittaranjan Colony and J block in Baghajatin, H block in Patuli, Rajdanga in Kasba, Nazirbagan in Dhakuria, parts of Santoshpur, Shahid Smriti Colony behind Peerless Hospital and Nonadanga near Ruby.

Mayor Sovan Chatterjee said: "The man was treated at a hospital and went back home. He died later but it is difficult to pinpoint the cause," Chatterjee said.

Chatterjee said efforts were on to find out the exact cause of the outbreak. "Until we find something, I should not comment," he added.

The death came on a day tests confirmed the presence of Escherichia coli bacteria in stool samples collected from patients.

Mayor Chatterjee said he didn't know about any such results of tests of samples.

The E.coli bacteria prompted doctors to suspect that leaks in sewer lines and drinking water lines are contaminating the potable water and leading to infections in parts of south Calcutta.

A relative said Biswajit was taken to Baghajatin General Hospital first around 10.30am. "The doctors asked him if he wanted to get admitted but Biswajit refused. The doctors administered three injections and gave some medicines. The doctors also advised ORS intake," said Ranjit Das, the relative.

According to Ranjit, Biswajit's condition improved. However, around 4pm, he complained of uneasiness and was taken to hospital by other relatives. "This time, the doctors at the Baghajatin hospital gave three injections and referred him to MR Bangur hospital. He died on the way to the hospital," said Ranjit.

Ajay Chakraborty, the director of health services, said: "Since he (Biswajit) was not admitted to the hospital, we are trying to find out records about his illness."

Death from diarrhoea doesn't happen so suddenly and within a few hours, he added.