Thousands of chickens were slaughtered in Hong Kong in 1997
More viruses, such as bird flu and Sars, could make the jump from animals to humans, scientists say.
The warning from scientists at a conference in London, came as the World Health Organization (WHO) expressed concern over the spread of bird flu.
Bird flu - or avian flu - has killed three people in Vietnam and experts are trying to establish if nine other deaths were caused by the virus.
Birds in South Korea and Japan have also been infected with the virus.
University of Hong Kong Professor Malik Peiris told scientists at the conference that more research was needed into the dangers of viruses in animals crossing over to humans.
"Most of these major emerging infection problems that we face are infections of animals jumping over to humans," he said.
"We really need to understand this animal-human interface much better."
Vigilance
The WHO also highlighted the risk of cross-species infections on Tuesday.
The bird flu crisis is affecting the poultry trade across the region
The transmission of the bird flu virus to humans was a cause for "heightened vigilance and surveillance", the organisation said in a statement.
It also said that 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 a new influenza virus "to which humans would have little, if any, protective immunity", the WHO warned.
Vietnam confirmed it had an outbreak of avian flu among chickens last week and it is thought that around one million birds have died, with thousands of others being culled.
Two more people in the country are also suspected of having the virus.
Scientists from the WHO have arrived in the country and more are on their way, along with an expert from the World Food and Agriculture Organisation (FAO).
Culls
South Korea also confirmed on Tuesday its first new case of the virus in a chicken for a week.
The Agriculture Ministry said the case was found at a farm in Yangsan, south-east of Seoul, and all chickens within three kilometres of the area would be killed.
After an outbreak in December, South Korea is thought to have culled up to two million chickens and ducks.
It is now thought that more than 6,000 chickens have also died from an outbreak of the disease in Japan's Yamaguchi prefecture in the south west of the country.
Thailand has declared itself free of bird flu, but it has admitted that it is trying to contain the spread of a strain of poultry cholera, which has forced it to destroy hundreds of thousands of chickens since November.
Hong Kong was the first place where bird flu was thought to have transferred to humans, after six people died from the virus in 1997 and 1998.