On the face of it, NSW Health is dealing with a needle in a haystack situation with this latest outbreak of legionnaires’ disease.

The Health Department is advising people who have been in the Sydney CBD area in the past 10 days to be on alert for symptoms of legionnaires’.

According to a statement from NSW Health, three women and four men, ranging in age from their 20s to 70s, are infected.

They independently visited locations in the CBD between Bathurst Street, Sussex Street, Elizabeth Street and Circular Quay in the 10 days prior to their symptoms.

The alert period takes in New Year’s Eve – when up to a million people flocked to the Sydney waterfront to see in the new year.

Given the timing, it’s not a stretch to say that hundreds of thousands of people have visited some of the spots of concern during the period.

It’s largely a matter of luck as to whether the outbreak infects a large number or a handful of people.

South-eastern Sydney acting public health director Professor Mark Ferson said anyone who had been in the area in recent weeks should monitor for symptoms.

“The early symptoms tend to be fever and chills, aches and pains, a cough – and if it progresses, shortness of breath is a sign that it’s turned into pneumonia,” he said.

“People can really go downhill from there. So it’s important that people developing those symptoms go and see a doctor.”

The ugly numbers

Public health officials are no doubt feeling some anxiety.

About one out of 10 people who get sick with legionnaires’ disease will die due to complications from their illness.

For those who are infected with legionnaires’ disease during a stay in a hospital, about one out of four will die.

The good news? You can’t catch legionnaires from other people, or very rarely.

What is legionnaires disease?

Legionnaires’ disease is a form of pneumonia that develops when a person breathes in the legionella bacteria.

As we recently explained when Jimmy Barnes became desperately ill with bacterial pneumonia, it is the most severe form of the lung disease.

That’s compared to lung infections caused by viruses or fungus.

The seven people infected so far are being treated with high-dose intravenous antibiotics, as was Barnes.

In his case, the drug failed to prevent the disease from spreading to his heart, and he was required to undergo emergency surgery.

How do you catch it?

As NSW Health explains:

There are many different species of legionella bacteria but the two that most commonly caused disease in NSW are legionella pneumophila (found in water) and legionella longbeachae (found in soil).

Legionella pneumophila bacteria can contaminate air-conditioning cooling towers, whirlpool spas, shower heads and other bodies of water.

Legionella longbeachae can contaminate soil or potting mix. People may be exposed to the bacteria at home, at work or in public places.

People catch legionnaires’ disease by inhaling small droplets of tainted water, but not from drinking it.

Outbreaks are more likely in the summer or early autumn, but they can happen at any time of year.

Hotels, hospitals, and large buildings tend to provide the ideal environment, if they have an air-conditioning system that uses water for cooling.

Signs and symptoms

According to an explainer from Medical News Today, signs and symptoms usually appear between two and 10 days after initial infection.

They typically include:

• A high fever, possibly over 40 degrees
• Chills
• A cough
• Muscle aches
• Headaches
• Confusion and agitation.

Often, the bacteria enter the patient’s lungs, leading to a persistent cough, shortness of breath and chest pains.

The cough may be dry at first, but as the infection progresses, there will be mucus and maybe blood.

About one in three infected patients will experience nausea, vomiting and diarrhoea.

Those with gastrointestinal symptoms will usually have an extremely reduced appetite.

Risk factors

Some risk factors can make a person more susceptible to legionnaire’s disease.

Ageing

It is more likely to affect people over 50 years old.

Existing health conditions

Prior conditions, especially respiratory issues, put people at high risk. These include smokers, patients with chronic obstructive pulmonary disease (COPD) and those with lung cancer.

A weakened immune system

Patients with diabetes, kidney disease, leukaemia and other cancers, and who are undergoing chemotherapy are at greater risk, as their immune system is compromised.

Lifestyle factors

Heavy alcohol consumption increases the risk.

There is some evidence that using cannabis may increase susceptibility to infection.

Diagnosis

The disease can be hard to diagnose at first, because its signs and symptoms may be very similar to other forms of pneumonia.

A physician will ask whether the patient has recently spent time in a large building, such as a hotel or hospital.

There are tests that can help to detect the disease, including a urine test, which is used to detect antigens. These are produced by the immune system to fight the legionella bacteria.

Imaging scans are used to check the state of the kidneys and lungs.

A lumbar puncture can reveal whether the disease has affected the brain.