New York City health officials are continuing to investigate a cluster of Legionnaires’ disease, a serious bacterial lung infection, where the number of cases has risen to 73, including three deaths.

Three people have died and at least 67 people have been diagnosed with Legionnaires’ disease in a cluster that broke out in New York City last week.

The disease is a type of pneumonia caused by the bacteria Legionella, which grow in warm water and can spread in building water systems such as showerheads.