Six people are receiving hospital treatment for suspected Legionnaires' disease, Public Health England said.
Eight cases were reported in West Bromwich last week and a single case reported in July has been linked to the "outbreak".
Three cases have been confirmed via lab testing, while six are suspected cases.
The Health and Safety Executive is investigating the outbreak, along with Sandwell Council and Public Health England (PHE).
Dr Adrian Philips, from PHE, said: "While we do not currently have a direct link between these cases, the evidence we have points to the possibility that there is a common source.
"We are taking detailed histories of the movements of the individuals to see if there are similar patterns which would help to identify a common local source of infection."
- A potentially fatal lung infection caused by legionella bacteria.
- Initial symptoms are "flu-like" include a high fever and muscle pain.
- It is treated by intravenous antibiotics.
- Legionella bacteria are commonly found in sources of water, such as rivers and lakes, which sometimes find their way into artificial water supply systems.
- NHS advice is that large buildings such as hotels, hospitals and office blocks are more vulnerable to legionella contamination because they have larger, more complex water supply systems and the bacteria can quickly spread.
- Legionnaires' disease is three times more common in men than women and mostly affects people aged over 50.
- An estimated 10% of people who contract the disease will die from complications arising from infection.
He said the disease was a rare, but potentially life-threatening illness which is caused by a bacterium commonly associated with water systems.
"As a precaution we are working with the Health and Safety Executive and Sandwell Council to identify and control any possible sources of the disease."
A Sandwell Council spokesman said: "The council is working closely with PHE and the Health & Safety Executive to find the source of this outbreak and we will continue to do everything we can to support the investigation."