Experts from around the world attending a Royal Society conference in London said there was an increasing likelihood of viruses jumping the species barrier and infecting human beings.
The observations come in light of the recent spread of SARS and bird flu across China and Southeast Asia.
China has confirmed two cases of SARS while the list of suspected cases has been mounting.
In Vietnam, three people have already died of bird flu while two more were confirmed to be carrying the disease on Wednesday.
While civet cats have been blamed for the spread of SARS in China, chickens and ducks have been the prime suspects in the bird flu cases.
China has undertaken a masive cull of civet cats and the Southeast nations have been destroying chickens and ducks by the thousands.
Civet cats have been blamed for
the re-emergence of SARS
Experts in Guangzhou in China, where the first case of SARS was recorded, said they were hopeful the cull would stop SARS from spreading.
They added SARS would always be around and early diagnosis and treatment were the only ways of containing it.
Scientists believe existing evidence point to the fact that SARS had jumped the species barrier from animals to humans.
The World Health Organisation (WHO) has also highlighted the risk of cross-species infection.
The transmission of the SARS and bird flu viruses to humans was a cause for "heightened vigilance and surveillance", the organisation said in a statement.
It also said the mingling of animal viruses with human ones could create opportunities for different viruses, previously specific to one species, to exchange genetic material.
This could create new viruses "to which humans would have little, if any, protective immunity", the WHO warned.
Vietnam announced on Wednesday it had two more suspected cases of bird flu as attention turned to pigs as possible carriers for the illness.
A 15-month-girl and a man are suspected of contracting avian influenza strain A, also known as H5N1, Dau Ngoc Hao, deputy director of the Agriculture Ministry's Veterinary Department in Hanoi, said on Wednesday.
Millions of chickens have been
culled in Vietnam and S Korea
They are among 15 people who are down with influenza in Hanoi and nearby areas. Twelve, most of them children, have died.
The WHO said on Tuesday that tests conducted by a Hong Kong laboratory had confirmed that bird flu killed three of the 15.
Hao suspected pigs were playing a role in the transmission of bird flu to humans.
"It is possible that the bird flu virus spreads from chickens to pigs before jumping on humans," he said.
Local officials said ducks and pigs were also dying in the southern provinces.
Hanoi declared last week that it had been struck by a fast-spreading bird flu that had hit other countries in Asia.
"The evidence to date is that there is no sign of human-to-human transmission"
Dr Shigeru Omi,
World Health Organisation
The WHO, however, said there were no signs yet of an outbreak.
"The evidence to date is that there is no sign of human-to-human transmission," Dr Shigeru Omi of the WHO said in a statement.
He, however, cautioned the consequences would be dire if the virus latched on to the human influenza virus and spread among people, who have little immune protection against the strain.
"The ensuing virus would then be highly pathogenic and transmissible," said Omi, the WHO's Regional Director for the Western Pacific.
In a region already reeling under the re-emergence of SARS, health officials say they are worried by the rapid spread of bird flu in Japan, South Korean and Vietnam.
The Paris-based world animal disease body OIE said it would send scientists to Asia to investigate the cases, which it said could pose a serious threat to humans under certain conditions.