I'm a native speaker, and my father practices Chinese Traditional Medicine, though I'm no expert on the matter.
From what I can gather, 火 is not a real thing, i.e it has no substance, rather than a general, collective and abstract terms for a set of human body reactions or symptoms commonly found when got a cold or infection. It often includes inflammation, irritation, bleeding, sore throat etc. Basically the symptoms when one's immune systems acting up. For that matter, I think westerns do 上火.
In China, the term is widely used among the general public even for those people who don't believe in CTM. And it is used when, as mentioned before one's immune systems is acting up no matter what the reason is. It could be caused by the invasion of external bodies like allergens, the reaction of external environments (e.g. 着凉), the accumulation of other conditions (e.g. abusive drinking or smoking), or simply a false alarm.
The thing is 上火 is often be used in such a manner as if it is the cause of something. However, it is actually the result, i.e. symptom.