An 8-year old child was brought to you for a high grade fever of 3 days duration with vomiting headache and decreased responsiveness. Examination revealed a distressed child , in shock with maculopapular and ecchymotic rashes on the skin with bleeding from venepuncture sites and hepatomegly. Investigation showed PCV = 65%, WBC 4000, platelet 25000 & mildly prolonged PT. CXR revealing left sided pleural effusion. Diagnosis is ?