The U.S. Centers for Disease Control and Prevention (CDC) just released a report following its investigation of a severe bacterial infection suffered by a dozen cruise ship passengers on two different boats.

Between November 2022 and June 2024, 12 people tested positive for Legionnaires disease after traveling on a cruise ship. A bacteria called Legionella causes “serious” pneumonia. Eight passengers tested positive after staying with one cruise ship while another four caught the infection via a different cruise line.

Of the 12 who suffered from the dangerous infection, 10 were hospitalized.

Thankfully, none died.

The CDC report doesn’t identify the cruise lines. Instead, it ascertains that samples from various hot tubs on board both ships contained “dangerously” high bacteria. The hot tubs were all positioned on private cabin balconies.

Per the CDC, “Hot tubs offer favorable conditions for Legionella growth and transmission when maintained and operated inadequately, regardless of location. Private hot tubs on cruise ships are not subject to the same maintenance requirements as are public hot tubs in common areas.”

Cruise Ship A reported four separate batches of LD cases to the CDC between December 2022 and March 2024. Investigations lead to more than 100 water samples. The CDC narrowed the source to hot tubs installed on private cabin balconies. It requested all 10 of Cruise Ship A’s private hot tubs be shut down for testing.

Six of the 10 hot tubs tested positive for Legionella. Of the six positive tubs, four of them tested “dangerously” high for the bacteria.

The CDC says this is the largest LD outbreak investigated since 2008.

“To meet CDC requirements, private hot tubs must only be shock-chlorinated, drained, and refilled weekly or between occupancies, whichever is sooner. Although the cruise lines adhered to current CDC requirements for operating and maintaining private hot tubs on ships A and B, these measures were insufficient to prevent Legionella growth.”

To remedy the outbreak, Ship A changed its protocols. It removed tub heating elements, drained the tubs between uses, and performed hyperchlorination and cleaning more often.

After remediation, the CDC continued testing hot tub water samples until August 2024, when zero Legionella were detected.

Cruise Ship B reported four cases this year.

Ship B communicated two in January 2024, another in February, and a fourth in May. They all required hospitalization. In turn, the CDC requested the ship close its eight private tubs for testing.

All eight hot tubs plus a potable water system tested positive for Lp1 and Lp2–14. Two tubs contained “high” levels of the bacteria.

The CDC said that the bacteria on Ship B emitted an aerosol, verified through environmental testing, that posed a risk to passengers nearby who didn’t even use the hot tubs.

The tubs remained closed until remediation and testing confirmed negative test results for the bacteria.

Ultimately, both cruise lines decided to only fill the private hot tubs upon guest request. They also say that they now drain and clean the tubs between uses.

In the report’s conclusion, the CDC recommends updating both public and private-use hot tub maintenance protocols to prevent Legionella growth and transmission.

In the meantime, if you’re staying on a cruise ship, in a hotel, or at an Airbnb with a hot tub, be sure to inquire about its cleanliness and that it was drained before your use. After this report, though, you might decide not to risk a possibly deadly bacterial infection in the first place.