A fourth child has fallen ill with a mystery virus that is suspected to have killed two children and left one in serious condition, health officials said Thursday.

The latest patient, a 3-year-old boy, was admitted to hospital Tuesday with fever and vomiting. His conditioned worsened after convulsions and he fell into a coma, the Centre for Health Protection said.

His condition was said to be critical Thursday with signs of blood poisoning and brain abnormalities.

Health officials are already conducting an investigation into the illnesses after the deaths last week of two girls, 9 and 3, who suffered from inflammation of the heart and blood poisoning.

A third girl, 7, is in critical condition, suffering from brain inflammation.

Thomas Tsang Ho-fai, controller of the Centre for Health Protection, said it was not known whether one single virus or different viruses were responsible and an investigation was ongoing.

In the meantime, he reminded parents to pay close attention to their children's health and seek medical help if they developed persistent high fever, vomiting or convulsions.

In March, the government closed all Hong Kong primary schools and kindergartens after the deaths of two children caused panic among parents who remembered the grim days of the 2003 outbreak of SARS, or severe acute respiratory syndrome, which killed 299 and infected 1,755 in Hong Kong.

It brought the return of surgical-style face masks, which were sold by the million and worn on the streets, in schools and in workplaces during the SARS outbreak.

An investigation into the March illnesses revealed the deaths were not the result of a virulent virus but strains of the H1N1 and H3N2 Brisbane virus, which had been circulating in different parts of the world earlier this year.

As well as SARS, Hong Kong saw the world's first modern-day outbreak of bird flu in 1997 when the virus infected 18 people, killing six of them.