A small number of Legionnaires' disease cases are being investigated in Rotherham to establish whether they are connected and whether there is a common source.
The UK Health Security Agency is working alongside Rotherham Council on the investigation.
Officials have stressed that the risk to the wider public is currently considered low.
They also point out that Legionnaires' disease is not usually spread from person to person.
Symptoms can include a high temperature, a cough, shortness of breath and chest discomfort.
Anyone concerned about symptoms should contact NHS 111 or their GP.
Source information
Source: UK Health Security Agency, Rotherham Council