Here is how I would approach your problem.

I would order a chest CT scan if you haven’t had one already. This gives much better detail compared to plain chest x-rays.

I also would review your old chest x-rays.

If the pneumonia always comes back in exactly the same area of the left lung, then the most likely cause is bronchiectasis. In this condition, there is damage to the airways (bronchi) in the lungs. This causes them to become wide and flabby.

You can be born with bronchiectasis or develop it later. This can happen after a severe lung infection or something else that inflames the lungs.

Once bronchiectasis develops, the flabby airway walls can’t clear mucus normally. So instead of it making its way up and out of the lungs, it sits there. Mucus that just sits in the airways is a great place for bacteria to grow.

These bacteria can easily move from airways into lung tissue. This starts a new bout of pneumonia in the same place where pneumonia was before. So if the left-sided pneumonia keeps returning to the same area of the lung, bronchiectasis would top my list of reasons why.

The chest CT scan and the old chest x-rays, however, might show a different pattern. For example, the pneumonia might have developed in different parts of the lung. And the CT scan could find there is pneumonia on the right side even though the x-ray didn’t show it.

Now, we need to consider other diagnoses. This means a longer list of possibilities, such as:

• An immune deficiency, such as a decreased ability to make antibodies to attack bacterial infections.
• Aspiration pneumonia. Saliva, fluids and food particles going down the trachea and into the lungs, rather than into the esophagus.
• An inflammatory condition, not an infection. Areas of lung inflammation from allergic reactions and autoimmune diseases can look like pneumonia in different parts of the lung.

Your doctor will typically order blood tests that may indicate a likely diagnosis. But you may also need either a bronchoscopy or a lung biopsy. Bronchoscopy is a procedure that uses a flexible tube with a camera to look inside the bronchial tubes, and take samples. The biopsy can be done with a needle guided by CT scan or with a small incision in between the ribs.