The fever was first reported in a small group at a slum cluster in Panposh on the outskirts of Rourkela, famous for its steel plant, Monday. Since then, it has rapidly spread to other parts of the town.
The high fever is accompanied by vomiting, headache and acute colic pain, said a senior district health official.
Most of the patients have been admitted to government and private hospitals.
Results of blood tests on about 300 patients indicate that it could be an airborne virus, though a few were also found to be suffering from malaria, the official said.
Specialists from the Veer Surendra Sai medical college hospital at Burla have rushed to Rourkela and are carrying out further tests, he added.