Lung diseases

TB is a lung disease caused by the bacterium Mycobacterium tuberculosis.

When someone becomes infected with this bacteria, immune system cells build a wall around the bacteria in the lungs.

This forms small, hard lumps known as tubercles.

Infected tissue within the tubercles dies and the gaseous exchange surface is damaged, so tidal volume is decreased.

Tuberculosis also causes fibrosis which further reduces the tidal volume. If the bacteria enters the bloodstream, they can spread to other parts of the body.

Common symptoms include a persistent cough, coughing up blood and mucus, chest pains, shortness of breath and fatigue.

Sufferers may also have a fever and many lose weight due to a reduction in appetite.

Many infected people are asymptomatic - they're infected but don't show symptoms as the infection is in an inactive form. They cannot pass on the infection but if they become weakened, by another disease or malnutrition, the infection can become active so they would start showing symptoms and be infectious.

It is transmitted by droplet infection - when an infected person coughs or sneezes, tiny droplets of saliva and mucus containing the bacteria are released from their mouth and nose. If these droplets are breathed in, others will become infected.
It tends to be more widespread in areas of poor hygiene and crowded conditions. 
It can be prevented with the BCG vaccine and treated with antibiotics.

Fibrosis is the formation of scar tissue in the lungs, as the result of an infection or exposure to substances like asbestos or dust.
Scar tissue is thicker and less elastic than normal lung tissue. This means that the lungs are less able to expand so can't hold as much air as normal - tidal volume is reduced. It's also harder to force air out of the lungs due to the loss of elasticity.
It reduces the rate of gas exchange in the alveoli because diffusion is slower across a thicker, scarred membrane.
Less oxygen is able to diffuse into the bloodstream, the body cells receive less oxygen, the rate of aerobic respiration in cells is reduced so less energy is released, leaving sufferers feeling tired and weak.
As well as fatigue and weakness, symptoms of fibrosis include shortness of breath, a dry cough, chest pain. Sufferers often have a faster breathing rate than normal, to get enough air into their lungs to oxygenate their blood.

Asthma is a respiratory condition where the airways become inflamed and irritated, usually because of an allergic reaction to substances such as pollen or dust.
During an asthma attack, the smooth muscle lining the bronchioles contracts and a large amount of mucus is produced.
This causes constriction of the airways, making it difficult for the sufferer to breathe properly. Air flow in and out of the lungs is severely reduced. So less oxygen enters the alveoli and moves into the blood. This means the body cells receive less oxygen and the rate of aerobic respiration in cells is reduced.
Symptoms include wheezing, a tight chest and shortness of breath. During an attack the symptoms come on very suddenly. They can be relieved by drugs (often in inhalers) which cause the muscles in the bronchioles to relax, opening up the airways

Emphysema is a lung disease caused by smoking or long-term exposure to air pollution - foreign particles in the smoke or air become trapped in the alveoli.
This causes inflammation, which attracts phagocytes to the area.
The phagocytes produce an enzyme that breaks down the protein elastin. The loss of elastin means the alveoli can't recoil to expel air as well. It also leads to destruction of the alveoli walls. This reduces the rate of gas exchange.
The lack of oxygen reaching the cells leaves sufferers feeling tired and weak. Other symptoms include shortness of breath and wheezing.
People with emphysema have an increased breathing rate as they try to increase the amount of oxygen reaching their lungs.

All diseases have risk factors - factors that will increase a person's chance of getting the disease. 
A link between two things is a correlation - however it doesn't mean causation, that one thing caused the other, as lots of other factors have to be taken into consideration.