CDC report warns private hot tubs on cruise ships may spread Legionnaires’ disease

Paying extra for a private spa on a cruise may feel cleaner than using a shared hot tub, but it does not necessarily mean you are less likely to encounter harmful microbes that thrive in warm water.

A newly published report from the US Centers for Disease Control and Prevention (CDC) cautions that hot tubs fitted to the balconies of cruise ship cabins could pose an even greater risk for spreading illness - not only to the people using them, but also to anyone who inhales the warm mist.

CDC findings on private hot tubs and Legionnaires’ disease

Following a recent investigation, the CDC says private hot tubs on cruise ships are the “likely source” of two recent US outbreaks of Legionnaires’ disease. The condition is a severe lung infection that spreads through inhaled aerosols and can trigger a life‑threatening type of pneumonia, particularly in older adults and people with weakened immune systems.

From 2022 to 2024, the CDC recorded 12 cases of Legionnaires’ disease across just two cruise ships. Ten of the 12 cases were serious enough to require hospitalisation. One ship accounted for eight cases by itself - the biggest single‑ship outbreak the CDC has investigated since 2008.

Why cruise ship hot tubs can harbour Legionella

Researchers have linked cruise ship hot tubs with Legionnaires’ disease outbreaks for decades. Warm tub temperatures are ideal for Legionella bacteria, meaning that if tubs are not emptied between users, thoroughly cleaned, and hyperchlorinated, conditions can allow a pathogen to spread.

However, unlike shared hot tubs, existing CDC guidance does not demand the same strict cleaning and operating standards for private hot tubs.

These recent outbreaks underline why that gap matters.

Balcony location may increase exposure for nearby passengers

Because many private hot tubs sit on cabin balconies, steam, spray, and splashes from the heated water can drift into the breathing space of neighbouring passengers - including people in cabins above and below - potentially carrying pathogens with them.

The first outbreak was documented in December 2022. Five passengers who boarded a ship in November later became ill with pneumonia. When officials sampled the cabins, heat exchangers, potable water tanks, decorative fountains, and public hot tubs, they did not detect any Legionella bacteria.

In August the following year, two further cases of Legionnaires’ disease were identified among passengers on the same vessel - this time in travellers who had access to a private hot tub.

Only after that did the CDC ask for all 10 private balcony hot tubs on the ship to be closed and tested. Legionella bacteria were found in six of the tubs, and four showed very high concentrations.

“"The hot tubs remained closed until their operation and maintenance protocols were modified and non-detectable Legionella sampling results were obtained,"” CDC officials write.

Even so, after later samples showed no bacteria, another passenger developed Legionnaires’ disease in March after being at sea.

In 2024, the CDC received reports of four more confirmed cases from a different ship.

This time, the CDC asked for all hot tubs on board to be shut and sampled immediately, including both private and public facilities. Legionella pneumophila was detected in all eight private balcony hot tubs on that ship.

No deaths occurred in either outbreak. But if the CDC is correct that private hot tubs were the source, current government guidance is not adequate to prevent future cases and should be updated.

“"Adapting public hot tub maintenance and operations protocols for use on private outdoor hot tubs can reduce the risk for Legionella growth and transmission,"” the report concludes.

The CDC report can be found here.