LONDON — There’s an ugly truth about the rubber duck, the popular bathroom toy that children put in their mouths and use to squeeze bath water into their siblings’ faces.

Something yucky is likely to be inside, scientists say: “potentially pathogenic bacteria” that can cause eye, ear and stomach infections.

A study by American and Swiss researchers found that toy ducks appeared to be a breeding ground for microbes. The murky water released from four out of every five ducks tested included Legionella along with Pseudomonas aeruginosa bacteria, often associated with infections acquired in hospitals, the authors of the study said.

The study, conducted by the Swiss Federal Institute of Aquatic Science and Technology, ETH Zurich and the University of Illinois, was published on Tuesday in the journal N.P.J. Biofilms and Microbiomes.