Doctors say the problem is that in a bid to save fuel they don't boil water before consumption. This, doctors say, is another reason for their recurring health problems.

'For them water-borne diseases and stomach infections are more common. With change in weather during monsoon, common cold and fever cases become rampant. We carry out preliminary tests and give them medicines. In severe cases, we refer them to the nearby government-run clinics or hospitals,' said Pradeep Bohra, a doctor with the Smile on Wheels mobile hospital van that camps in the slum once every fortnight.

Another doctor, David Singh, said: 'For a swine flu test all we can do is to refer them to a testing centre. But we can't ensure that they go there. So sometimes we give incentives - like a pain killer for a headache and when they see it works they'll listen to us.'

At another slum, while some are a little aware about the spread of the virus, they don't know how to tackle the situation.

'We saw the TV news that some flu is spreading. We can't tell the difference from the common flu - what should we do?' said Chhaya, 35, a resident from Nehru Camp in south Delhi's Govindpuri area.

The Municipal Corporation of Delhi estimates that more than three million people reside in about 1,000 unauthorized slum clusters in Delhi. Officially, 14.82 percent of Delhi's 16 million-plus population is estimated to be below the poverty line.