Mm. Invisible to our sight, at least. Although if one has a certain type of high powered lens, you'd be able to see them.

Herbs are good for symptoms, yes. I myself find that chicken soup and tea is good for the cold but... as for killing the cause of the illness, it won't do much damage. [ Wait... ] Although one fungus has been known to do as much. Even the ancients knew how to use it as an antibiotic, can you believe that? Today it's called penicillin. [ And then thinking aloud: ] I do wonder if they have any here in the tower.