At 3.30am this morning (Saturday) Surrey Police were called to an address at Thameside, Chertsey following a request for assistance by the ambulance service.
The occupants of the property, two adults and a child had been taken unwell and were taken by ambulance to hospital.
A seven year old boy was later declared dead at the hospital. A man in his 40's and woman in her 30's remain in hospital in a serious condition which at this stage is not thought to be life threatening.
A specialist team from the fire service were deployed to the scene to investigate the circumstances.
Nearby properties were evacuated as a precaution and the occupants from the adjoining properties were advised to go to hospital to be checked out as a precaution.
Officers visited premises within the area to reassure local residents and gave the advice to stay indoors whilst the investigation is underway.
Fifteen people including police officers were taken to hospital as a precaution. One person who showed signs of illness and has now been released from hospital.
Chief Superintendent Dave Miller said: "It is very sad that a young child has died earlier today - our thoughts are with his parents who are currently ill in hospital. Police officers are currently with them and we will continue to offer our support during this difficult time.
"Our aim is to keep people safe and find out the cause. I would like to reassure local residents our officers are working closely with partner agencies and a full and thorough investigation is currently underway to determine the circumstances around this tragic incident.
"We are currently following various lines of enquiry and it would be wrong to speculate on the cause of death at this stage.
"The cause of the illness is currently unknown but what I can confirm is that the people who were taken ill are from a very localised area.
"We cannot rule out that there may be a link to flooding in the local area and whilst the investigation is ongoing local residents should follow sensible precautions.
"At this stage of the investigation we do not believe this is a widespread incident; there have been no further reports of casualties, and we have taken advice from medical experts who have indicated that it is likely to be a localised event."
Anyone in the area local to Thameside, Chertsey who are feeling unwell and showing the signs of vomiting, diarrhoea and fever are advised to seek medical assistance.
A police scene guard remains at the location and road closures to Thameside are currently in place whilst the investigation continues.
The coroner's office has been informed. An update will be issued in due course.
