From the description of the chest x-ray there are a few possibilities. 1st is that what is being seen (the vague density) is actually nothing. This is common. It may simply be a bad x-ray technique or some shadowing from something else. It may also be some areas where the lung has collapsed a little, such as from the film being taken after one has held their breath for too long or during breathing out.
If anything, I would tend to agree that them most it would be is an early pneumonia. A tumor or other more fixed/solid object would tend to be a more defined area on the x-ray. This area was not seen on the side film, only on the frontal film. It would be incredibly unlikely for a tumor to do this. Actually, it would be really unlikely for anything real to show up only on the frontal x-ray and not on the side, including pneumonia.
If you are having the CT scan solely for this, I would recommend first repeating the chest x-ray to see if the issue has resolved. If there are other reasons, the CT scan will give an excellent look at the lungs to see if there is anything, such as a pneumonia, actually there.
Regarding the question of infiltrate vs. density this is a common question. By strict definition, infiltrate is a diagnosis that must be made under a microscope. It refers to cells invading the area (inflitrating the area). The proper term in radiology is opacification or haziness. X-rays can only differentiate different shades of x-ray penetration. There is no way to know that something is an infiltrate. However, infiltrate is a very commonly used word in radiology often to indicate an infectious rather than some other process.