Dr Veronica Chan, chairman of the microbiology and parasitology department at the University of the Philippines' College of Medicine, said human beings would have no protection from the resulting new strain of flu.
"There is no protection from that new strain of influenza virus, so it's going to cause a big epidemic," she said.
"The pandemics that occured in the previous century, the 20th century, were really devastating, especially the Spanish flu. We had that in 1918 and 40 million died of that. We should worry. It kills. It kills."
At least three people have died of bird flu in Vietnam while two more suspected cases emerged on Wednesday.
They are among a group of 15 people who fell ill with influenza in Hanoi and surrounding areas.
"In the SARS experience, many more survived than died of it. That means the immune system worked very well against the SARS virus"
Dr Veronica Chan
University of the Philippines' College of Medicine
Twelve, most of them children, have died.
The World Health Organisation (WHO) said on Tuesday tests conducted by a Hong Kong laboratory had confirmed that three of them had died of bird flu.
Chan said a new virus could be born if a person already carrying a strain of influenza caught bird flu.
"The bird and the human influenza can re-assort their genetic components and come out with progenies, meaning products or a new virus," she said.
"If the influenza occurs in a pandemic form, which means that the global population has no defence against it, then we worry.
"In the SARS experience, many more survived than died of it. That means the immune system worked very well against the SARS virus."
Severe Acute Respiratory Syndrome infected 8000 people in about 30 countries early last year, killing more than 700.
It has re-emerged in southern China with two confirmed cases being reported in the last few days.
Several more people are suspected to be carrying the virus and are being monitored.