Your opal creature uses silicon for its body, but has conventional carbon energetics.
When we eat, we use our food for two things. We metabolize some for energy and we use some for anabolic processes - building our bodies
There are creatures who split these functions. Iron oxidizing bacteria, for example, have bodies made of the same sorts of carbon molecules that we use. But for energy, they oxidize iron. They do not use the products of their energy metabolism for any anabolic process - the iron oxides are waste.
Your creatures use silicon to form their bodies. They are opalescent creatures, their bodies laid down from layers of soluble silica. Energetics however are much like ours - they oxidize oxidizable carbon and retrieve the energy.
These are creatures of the deep earth where they have lived for ages. Some were accidentally retrieved. Their silica bodies make them unattractive as food, and so they colonized the upper worlds. Free in the environment there is much competition from bacteria and other organisms for reduced carbon. In the oceans the radiolarians and other silica-using creatures compete for soluble silica but on land there is no competition.
But the opal creatures are immunologically invisible. Our immune systems are long habituated to particles of sand and so these creatures provoke little reaction. Esconced in the bodies of top world plants and animals, the opal creatures are safe and also provided with a consistent supply of food. The issue for them is still soluble silica, which is present inside and outside of us but at very low concentrations. The low concentration of silica limits the growth of these creatures which also limits how morbid and dangerous the infections are. Most top land creatures are chronic carriers of the opal creatures, and come to little harm from them.
Actually carriers of these tiny opals can look pretty cool, because the opal creatures are pretty.