Fluid in the lungs, which is also referred to as pulmonary edema or lung water, is an illness which should be cured by a doctor if it cannot be treated at home. Once the air sacs are packed with water, the body can no longer absorb oxygen from the lungs. Find out more details on http://www.ehow.com/about_5526736_causes-water-lungs.html.
Fluid in the lungs, or in some cases it could be pus, is also a tale-tale sign of pneumonia - which is a VERY dangerous illness despite how 'common' it has become. It's important to see a doctor immediately because your oxygen levels could drop to fatal levels before you even knew anything was seriously off.