CHINA'S largest city, Shanghai, has been hit by an outbreak of cholera which has claimed at least seven lives and infected about 200 others, according to doctors in the city. The outbreak has been largely confined to shanty towns on the outskirts of the city occupied by migrant labourers. But some doctors fear it could spread to the rest of the city unless preventive measures are taken immediately. ''Cholera can spread very quickly, especially in densely populated areas like Shanghai so we need to control this outbreak right away,'' a doctor at one of city's main hospitals said. However, the municipal government appears to have done little to control the potentially fatal disease. There have been no public announcements of the outbreak and doctors have reportedly been told not to inform patients that they are suffering from cholera, known as ''Number Two disease''. There have been unconfirmed reports that the city is considering demolishing some of the migrant labourer neighbourhoods where the disease is prevalent. Sanitation in these neighbourhoods is poor and people live close to each other, making the area an ideal breeding ground for cholera, doctors said. The Shanghai Public Health Bureau yesterday refused to comment on the outbreak or government plans to control it. Despite the official secrecy, news of the outbreak is common knowledge in Shanghai. Most people are not over-concerned, claiming that if individuals take the right precautions they will not be affected. Shanghai residents are used to epidemics of infectious diseases such as hepatitis and are generally familiar with the precautions necessary to avoid infection. Some doctors fear Shanghai residents are being too complacent about the disease, thinking it only affects migrants.