Shayuan market in the southern Chinese city of Guangzhou is typical of hundreds of thousands of such places across the country - fruit, vegetables and live chickens are on sale sided by side.

There is blood, excrement, spittle - and amid them people who want their poultry to be freshly slaughtered.

These markets have been called breeding grounds for viruses.

"Once the virus gets into the market, it stays," says Malik Peiris, a virologist at the University of Hong Kong. He explains that the virus spreads from one bird to the next, because there are always new birds arriving to replace those which are slaughtered. He says a market is an ideal environment for a virus which is stored and amplified.

Easily spread, easily stopped

The latest H7N strain of the bird flu virus that has already killed over 30 people was probably also spread by the markets.

Traders transport live birds from market to market, which means that cages and boxes with infected excrement do the rounds.

In the same way it is easy for the virus to spread, it can also be easily stopped. Since the Shanghai authorities banned the sale of live birds at the beginning of April, there has been a decrease in infections and no human has contracted the virus.

Nonetheless, it is not so easy to close down all the poultry markets in China. That's why Peiris suggests closing them for one or two days a month to interrupt the chain of infection. Another measure would be to ban live birds from being held overnight.

He says that this has clearly prevented the spread of disease in Hong Kong.

The Chinese government has not yet issued any new regulations for poultry markets. However, experts think that the sale of live birds might remain banned in big cities such as Shanghai.

The World Health Organization has said there is no evidence that the new strain is easily transmissible between humans.