Katharine tells Lorien about the  worst thing you can get in the mail. 

Sources:

https://www.cdc.gov/anthrax/basics/index.html

https://en.wikipedia.org/wiki/2001_anthrax_attacks

https://www.fbi.gov/history/famous-cases/amerithrax-or-anthrax-investigation

https://www.propublica.org/article/new-evidence-disputes-case-against-bruce-e-ivins

https://www.cdc.gov/anthrax/bioterrorism/threat.html