[Emerging Infectious Diseases] [Volume 4 No. 4 /October-December 1998] Please note that figures and equations are not available in ASCII format; their placement within the text is noted by [fig] and [eq], respectively. Greek symbols are spelled out. The following codes are used: (ft) for footnote; (sup) for superscript; (sub) for subscript; >/= for greater than or equal to. URL: ftp://ftp.cdc.gov/pub/EID/vol4no4/ascii/filename.txt URL: ftp://ftp.cdc.gov/pub/EID/vol4no4/adobe/filename.pdf URL: ftp://ftp.cdc.gov/pub/EID/vol4no4/ps/filename.ps