'Fibrosis' is the medical term for scarring. Essentially, excessive damage happens to a certain area - beyond which the body can replace with normal tissue - and instead it deposits fibrin (the substance that makes up a scar...picture tough webbing). Scar tissue is inherently rigid.

Scarring on the skin doesn't cause much hassle (other than cosmetically) as the skin doesn't need to be particularly stretchy. Unfortunately, this is not the case when it comes to the lungs. Fibrosis of the lung makes the airways very taut and unforgiving - the dense scaffolding does not allow the alveoli to expand seamlessly during inhalation. 

The tough scar fibres reduce the ability of the airway/alveoli to expand. Thus, lung fibrosis is, what we call, a restrictive lung disease. A restrictive lung disease is one in which the condition makes efficient inspiration the main issue. An obstructive lung disease is one in which the condition makes efficient expiration the main issue - COPD, asthma, etc. 

When a patient has ongoing difficulty breathing, they will be sent for pulmonary (lung) function tests. It is here that the doctors may find a reduced lung capacity. This points towards a restrictive disease of the airway. During these tests, the doctors will also measure the diffusion of carbon monoxide across the alveoli wall. 
- Yes, gibberish, we know. Basically, it tests how well your lungs allow the transfer of gases in and out of the bloodstream. In lung fibrosis patients, the 'DLCO' can be prolonged in an abnormal manner. Not only has the scarring made it more difficult for the airways to expand, it can also impair the gas exchange at the microscopic level. 

What causes lung fibrosis? There are many causes of lung scarring.

- Sometimes, we can point to a person's occupation. For example, exposure to nasty chemicals or asbestos in the air over long periods of time.

- Sometimes, we can point to previous cancer treatments (such as targeted chest radiotherapy - quite rare). 

- Medications such as methotrexate and amiodarone can cause scar tissue to form in the lungs. 

- Chronic autoimmune conditions can lead to inappropriate levels of inflammation and scarring (eg. Sarcoidosis, Wegener's Granulomatosis). In a similar vein - lingering infection can lead to cycles of inflammation and scar formation.

The most common cause of lung fibrosis, however, is idiopathic pulmonary fibrosis. 

Fibrosis - Scar formation. Pulmonary - Lung. Idiopathic - We don't know. 

Yes, unfortunately, the exact driving force of irregular scar tissue formation is unknown. We believe there are complicated genetic factors at play, and possible unknown environmental triggers in certain individuals. 

How do we treat this?

Lung fibrosis is a very tricky thing to solve once the scars form. Our best line of defence is prevention. The earlier we diagnose the disease, the earlier we can try and treat the underlying cause. For instance, immunosuppressant medications may be considered in autoimmune conditions such as sarcoidosis. 

There is a very niche, pharmacological class of drugs known as the anti-fibrotics. Examples of such are nintedanib and pirfenidone. In idiopathic pulmonary fibrosis, these medications can often slow the rate of disease progression.