Thanks Roger - a ghost moth caterpillar! And it did throw itself into violent contortions when I picked it up - although I didn't see/hear any snapping of jaws :)
Yeah: caterpillars get infected with viruses and bacteria just like we do, with flu, measles, etc (probably not covid), and they don't have a GP to go to, to get antibiotics.
Yes. Most Hepialidae larvae are innately programmed to burrow into something, and to pupate in their burrow. They do not normally walk about in broad daylight where any passing bird can pick them off.