無詞性毒信 (內有有毒物質或被有毒物質,尤其是炭疽病毒,感染的信件)。
Greg Poland, a professor of medicine and infectious diseases at the Mayo Clinic in Rochester, Minn., said that although "no envelope is airtight" and any powder inside could be squeezed out in processing, the probability of a significant number of anthrax spores escaping is low. ... So far, he said, no anthrax cases have been caused by what he called "collateral mail." (The Washington Post,華盛頓郵報)