[creamyhorror]

It's because they're not only talking about everyday visible inflammation. My understanding is that "inflammation" is a phenomenon in which cells release chemicals telling other cells that there's an injury, pathogens, or toxins nearby. These chemicals cause other cells to release further chemicals (a "signalling cascade"), leading to all sorts of effects ranging from the familiar swelling and redness and summoning of immune cells, all the way to allergic reactions, neural effects, long-term health outcomes, and (apparently) increased social media use. The effects no doubt differ based on the types and levels of inflammation-signalling compounds being released (of which there are many).

The overall level of inflammation is commonly determined by measuring the level of C-reactive protein in blood (it rises as a result of inflammation). It's commonly measured in blood tests.

There's a natural role for inflammation in the body, but it's also possible to have too much inflammation and experience negative outcomes as a result. A lot of recent research has been about the effects of inflammation signalling throughout the body, and much of the interest in antioxidants is because they may serve to reduce oxidative stress (which damages cells and also comes about as a result of inflammation).

"Chronic low-grade inflammatory process, on the other hand, plays a central role in the pathogenesis of a number of chronic diseases." - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4736408/