Health officials are trying to identify the source of a Legionnaires' disease outbreak which has infected six men.
Health experts say they have so far failed to pin down its cause.
The only link at this stage is that all the men have visited Norwich city centre within the past month, a Health Protection Agency (HPA) spokesman said.
The men, aged from mid-30s to 70, have all responded to treatment at the Norfolk and Norwich University Hospital with two being discharged.
The HPA spokesman said: "As these people all live in the Norwich area, this may not be significant.
Inhaling mist
"We may not find a common cause and it may not have anything to do with Norwich, they may have got it from somewhere else."
He said the men do not work together, do not have similar occupations and do not visit the same gym.
He said cooling towers in the Norwich area had been chlorinated as a precaution, but early tests suggested these were not the source.
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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