With regards to the term "pneumonitis," it refers to inflammation of lung tissue and generally is used to describe findings that are non-infectious, caused most often by an allergic response, as opposed to "pneumonia," which would be caused by an infection with some microorganism.
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Majority of the times the X-ray is negative due to a viral cause, however there can be mild interstitial disease usually viral in etiology. If there are any relevant findings, they can be a bacterial cause which is treated more aggressively. Typically these resolve over time, however a follow up may be suggested prompting additional assessment. Typically, the chest X-ray suffices but the dominant approach is to assess the child clinically. There are some remote outliers that are more serious, either acquired or congenital. I would not put weight on the overall finding, but more on the final outcome.