Anthrax is read as a hidden danger arriving in something ordinary, like a letter or powder. Tradition sees it as distrust and the sense that a threat might lurk where you least expect it.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.