The mysterious disease, affecting children in West Bengal's Murshidabad district, has claimed 20 more lives taking the death toll to 39.
The children, all between the age group of one to six, died in different blocks of Jangipur sub-division during the past couple of days, district health department sources said on Wednesday.
Five children were now being treated in Jangipur hospital.
Earlier, 19 deaths were reported from Lalgola area of the district on Saturday.
Although the disease was yet to be identified, the common symptom noticeable in all children was high fever, the sources said adding blood and stool samples have been sent to Kolkata for examination.
Medical camps have been set up in both the areas.
The Director of Health Services Prabhakar Chatterjee, who toured the affected areas over the weekend with a team of experts, told reporters in Kolkata on Tuesday that malnutrition, heat pyrexia and electrolyte imbalance were being considered as possible causes.