Fibrosis refers to scar tissue that has replaced healthy tissue. This is what happens in the lungs of people with pulmonary fibrosis. Inflammation (swelling) in the lungs usually happens before or at the same time as the formation of scar tissues.
There are several substances known to cause lung fibrosis, but people often develop lung fibrosis even when there is no apparent cause. When the cause is unknown, it's called idiopathic.
Idiopathic pulmonary fibrosis is a serious condition whose cause is not well understood. Another condition very similar to idiopathic pulmonary fibrosis can happen in some people with certain diseases, especially autoimmune diseases like systemic lupus erythematosis or scleroderma. Whether this other condition is the same thing as idiopathic pulmonary fibrosis or slightly different is unknown.
When pulmonary fibrosis is idiopathic, it most often occurs in people 50 years of age and older, but people of any age can develop it. Pulmonary fibrosis can be detected at an early stage or late stage but usually gets worse with time. Sometimes it progresses slowly but it can also progress quickly over just a few years or even months.