Doctors previously thought the Mycobacterium abscessus bacteria could only be caught from water and soil. But hospitals around the world may now have to change the way patients are treated, the study says.
Around 3-10% of cystic fibrosis patients in Europe and the US are infected with the hard-to-treat bug.
There are around 9,000 people with cystic fibrosis in the UK although around one-in-25 people carries the faulty gene which causes the condition.
It affects the internal organs, especially the lungs and digestive system, by clogging them with thick sticky mucus which makes it hard to breathe and digest food.