Baby Clementine was burning with fever, coughing and sleeping almost around the clock.

Her parents, Emily Ritchie and Lachlan Barnes, had no idea their 15-month-old had fallen ill with the flu.

Like more than two-thirds of Australian parents, they hadn't immunised their daughter against the potentially deadly virus.

The Royal Children's Hospital, Melbourne has seen an alarming increase of almost 50 per cent in flu cases so far this season compared with last year.