Four people have fallen ill with the potentially-deadly bacteria in the Grimsby area.
Investigations have failed to find the source of the infection but experts say all four victims have been in the area around Freeman Street before falling ill.
Simon Padfield, consultant in communicable disease control at Public Health England, said inquiries were being centred on the area. All four patients have responded to treatment and GPs have been urged to be vigilant for the symptoms.
Cooling towers are often linked with the spread of legionella bacteria and additional disinfection measures have been put in place at sites in the town.