The World Health Organization issued further warnings of a deadly flu pandemic today as a Vietnamese woman became the sixth person to die from the bird flu in that country in the last three weeks.

WHO officials fear that the bird flu virus, also known as H5N1, may be mutating in ways that would make it difficult to treat and cure. The WHO's latest report on preparedness for a flu pandemic warned that this next outbreak 'may resemble that leading to the 1918 pandemic' of Spanish flu that claimed over 40 million lives world wide.

Thus far, there is no evidence that the bird flu is being spread from direct person-to-person contact. Most of the 38 people who have died this season from the flu had direct contact with infected fowl. The WHO has urged all countries to work on an effective vaccine before person-to-person spreading of the disease takes place.

Health officials fear that there has been an increase in the animal hosts the disease has been found in. In addition to birds, domestic cats and captive tigers are now susceptible to the virus according to a report in Reuters.

The last flu pandemic took place in 1968 and caused between one million and four million deaths world wide. According to the WHO, the next outbreak is long overdue. If such an outbreak takes place, the best case scenerio is 2 million to 4 million deaths worldwide according to WHO experts.