DATE OF ANTHRAX ONSET BY FORM OF TRANSMISSION FROM LETTER TO PATIENT

For most of the 22 cases, the anthrax spores were transmitted directly from a contaminated envelop to an infected person.  For four persons, however, the transmission was likely indirect, going from contaminated envelop to other mail or site to the infected person.  Such indirect transmission when involving other mail is termed "cross-contamination."  Indirect transmission by nature is more difficult to demonstrate than direct transmission. Among the four indirect cases, evidence of cross-contamination remains problematic with case 22.