Figure 1.

Schematic of the matrix model of mailborne transmission of anthrax spores, based on cross-contamination. N letters entering node 1 (e.g., mailboxes), each with >10¹⁰ spores, cross-contaminate letters at nodes 2, 3, … , which in turn cross-contaminate additional letters in successive generations. The coefficients c[m]_jk of the contamination matrices indicate cross-contamination from node m and spore class k to node m+1 and spore class j. The model can be extended to additional nodes and additional classes of spore counts.