The doctor, whose clinic is situated in an upscale residential area, treated more than 200 cases of typhoid fever in the first week of September. Residents suspect their housemaids and cooks to be the carriers of the disease, which is transmitted through the ingestion of food or drink contaminated by the feces of an infected person.

A distraught father whose daughter contracted typhoid muttered in Dr. Chakraborty’s clinic, “The whole country eats excrement?”

The doctor weighed the question with more seriousness than he had expected, and said that that was probably true. On the brighter side, she said, it contributed to the eradication of polio in India. People with the best access to the oral polio vaccine discharged it in their stools, allowing the vaccine to reach the drinking water of those on the outer fringes of society.

But in the exchange of germs and maladies between the rich and the poor — a perpetual transaction in Indian society — there is a disturbing trend. The poor live in conditions that make them effective carriers and transmitters of diseases, which the rich then combat with excessive use of antibiotics that are easily available over the counter. This leaves the microbes increasingly resistant to the most powerful drugs of Western medicine, allowing them to transmit the enhanced diseases back to the poor.

In the case of dengue, the poor alone cannot be blamed. The predominance of the disease is, in a way, a result of a private-public partnership. As the Aedes aegypti mosquito needs fresh water to breed, it thrives in fancy homes and construction sites as well as in the slums.

There is not yet a cure or vaccine for dengue. But now, scientists from the New Delhi division of the International Center for Genetic Engineering and Biotechnology, a global research organization, have reported that a vaccine they have developed has shown encouraging results on mice. Dr. Navin Khanna, who is leading the team of researchers, and whose daughter, by coincidence, recently tested positive for dengue, told me, “It will take at least another five years before the vaccine is ready, if everything goes according to plan.”

He is among the scientists who are unable to make sense of the Indian government’s poor investment in research on mosquito-borne diseases. “The push is coming from the West,” he said.