As the Sydney flu spreads, so do medical fears of a worst-case scenario - that it will combine with the bird flu and mutate into a deadly super-virus that sweeps through humans. Virologists and researchers said that while it was highly unlikely, someone suffering the Sydney flu could also contract the bird flu and become an incubator for a whole new strain. A new avian flu could be created with different symptoms and characteristics - including the ability to jump from person to person for the first time. An earlier mutation saw the avian flu H5N1 jump from poultry to humans - a jump discovered after a three-year-old boy died of respiratory problems and a rare form of brain inflammation at Queen Elizabeth Hospital in May last year. The avian flu strain H5N1 is feared to still linger despite the lack of new cases since December, say experts, and could soon return. H5N1 is already feared for its ability to kill by swiftly shutting down vital organs. Seven out of 18 people died in the Hong Kong outbreak last year, all of whom are believed to have caught it directly from birds. Chinese University virologist Professor John Tam Siu-lun said: 'This is the thing we're worried about. If we don't stop the bird flu going around now, [and] if we have two viruses infecting the same person all mixed up together . . . [it can mutate into] a hybrid virus which can spread around humans very rapidly.' Avian strains have mutated wildly before but never to the human-to-human stage. A mild outbreak in Pennsylvania chicken farms in 1983 evolved to something more deadly with six months. It started by attacking the respiratory and intestinal tracts of birds. Later victims died very quickly as it ruptured internal and external blood vessels.