Illegal aliens who would have never been immunized.
Also, I found this saying that routine smallpox vax ended in 1971. I know they were being given out after that though because I have the scar myself from that particular vax. Theoretically anyone that didn't get it is vulnerable though.
https://publications.aap.org/pediatrics/article-abstract/49/4/489/47117/THE-END-OF-ROUTINE-SMALLPOX-VACCINATION-IN-THE?redirectedFrom=fulltext