Barnaby E Young1,2,3, Annelies Wilder-Smith3,4. 1. National Centre for Infectious Diseases, 16 Jalan Tan Tock Seng, Singapore. 2. Department of Infectious Diseases, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore. 3. Lee Kong Chian School of Medicine, Nanyang Technological University, 11 Mandalay Road, Singapore. 4. Department of Disease Control, London School of Hygiene and Tropical Medicine, Keppel St, Bloomsbury, London, UK.