text: my mother has water behind her lungs what causes that ?   if she has fluid it wouldn't be behind the lungs but in them. ordinarily the lungs fill with fluid because the heart is flabby and is not pumping correctly so fluid backs up into the lungs and a stated is called chf, congestive heart failure. pleurisy on the other hand is an infection in the pleural space, between the layers and is treated with antibiotics. it causes chest type pain. a course of antibiotics will cure this but it does not take months to happen.