Indian health officials say they are growing alarmed at the spread of HIV to rural areas where health care is poor.
India is home to one in seven HIV-positive people
The warning from National Aids Control Organisation head Sujata Rao came on the eve of World Aids Day.
Meanwhile, the government has stood by official figures which show India's rate of HIV infection falling sharply.
More than 5.1m people have the HIV virus in India, official figures show. Only South Africa, with 5.3m, has a higher number of cases.
"What is really worrying us is that the epidemic is really getting into the rural areas," Ms Rao told reporters.
"Until now we thought HIV/Aids was an urban phenomenon," she added.
People 'missed out'
Health Minister Anbumani Ramadoss defended India's official HIV figures after UN Aids chief Peter Piot said they were wrong.
Mr Ramadoss said figures showing just 28,000 new HIV infections in 2004 - down from 520,000 the previous year - had been verified by the World Health Organization.
But he conceded that they may not tell the whole story.
"I am sure there will be people missed out in Bihar, Uttar Pradesh, Madhya Pradesh, Gujarat, Punjab, Rajasthan and the north-east," he said referring to some of India's largest, remotest and most populous states.
Voluntary groups and UN body UNAids say the rate of infection is far higher in India than the government says it is.
Earlier in November, Mr Piot said: "India having only 28,000 new infections is plainly impossible."