Three people have died and dozens have become infected after being diagnosed with legionnaires' disease during an outbreak in Milan.

Health officials are scrambling to contain the spread, which they fear has contaminated the local water supply.

Legionnaires' disease is a potentially fatal form of pneumonia and everyone is susceptible to infection. The risk increases with age but some people are at higher risk including people over 45.

Around 90 percent of people exposed to legionella bacteria, which can grow and spread in built water systems like cooling towers and pools, do not experience symptoms.

But up to 15 per cent of those who do become ill can suffer from severe disease, which can lead to life-threatening complications.

The latest figures from the European Center of Disease Prevention and Control (ECDC) show at least 49 cases of the disease have been recorded in the Lombardy region.

The first recorded case was on April 11.

All cases are between the ages of 26 and 94, with 28 women and 21 men involved.

The majority (45) had some sort of underlying health condition, which put them at higher risk of serious illness.

About 12 of the 49 confirmed cases remain in hospital, while three have died, the ECDC says.

According to the Centers for Disease Control and Prevention (CDC), the most common way for someone to get sick is by breathing in mist containing legionella - the bacterium which causes legionnaires' disease.

The bacteria can become a health concern when they grow and spread in human-made building water systems like:

Showerheads and sink faucetsHot tubsDecorative fountains and water featuresHot water tanks and heatersLarge, complex plumbing systemsCooling towers