Health officials have confirmed two new cases of Legionnaire's disease in the Norwich area, making eight in the past few weeks.
Six cases were identified earlier this month and so far health experts say they have failed to pin down its cause.
"The only link at this stage is that they all made visits to the city centre in August," a Health Protection Agency (HPA) spokesman said.
The patients, aged from mid-30s to 80, have all responded to treatment.
Inhaling mist
"As far as we know there is no proven link, the only common factor is that the group live in and around Norwich," the spokesman added.
Public health bosses have traced the movements of those who contracted the disease and checked all potential sources of the outbreak - such as cooling towers and water fountains.
Of all the samples taken at potential sources, 60% have been returned and all are negative.
Legionnaires' disease is most often contracted by inhaling mist containing legionella bacteria from water sources such as whirlpool baths, showers and cooling towers.
It cannot be passed from one person to another.
The most common cause of the disease is contaminated air conditioning systems.
About 300 to 500 cases are reported each year in England with half associated with travel abroad.
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