A new batch of cases of the bird flu in Vietnam have experts worried that the disease has already mutated into a form that can be spread directly between humans.

The World Health Organization has warned that if and when the bird flu is transmittable directly between humans, a worldwide pandemic could occur, resulting in the deaths of tens of millions of people worldwide.

The instance that most concerns WHO officials took place in the province of Thai Binh in northern Vietnam. An entire family contracted the disease as did two nurses who were caring for them. It is believed the two nurses contracted the deadly virus directly from their patients.

The WHO released a statement on Monday saying, 'Such cases can provide the first signal that the virus is altering its behavior in human populations and thus alert authorities to the need to intervene quickly.'

47 people have already died in Southeast Asia from the H5N1 bird flu virus. WHO officials fear this could be the biggest and deadliest outbreak of influenza since the Spanish flu epidemic of 1918-1919 which killed between 20 million and 50 million people worldwide.