Figure 2. Infection pattern during pneumonic plague outbreak, northern Madagascar, 2011. The outbreak spread to other neighboring villages during January 14–February 9. Twenty persons in 6 households (A–F) in 5 villages had symptoms of pneumonic plague. The outbreak population was divided into 3 groups (group 1: case-patients 1–5; group 2: case-patients 6–17; and group 3: case-patients 18–20). Patients received treatment by January 28. Because of geographic distance, none of the patients in group 3 received treatment. Contacts were divided into family contacts (c1–c16) who lived in an affected household and other contacts (c17–c41) who interacted with infected patients or patients who died. All contacts, except c10 and c25, received antimicrobial drug prophylaxis. Two contacts (c14 and c25) were seropositive (single serum sample); all other contacts remained seronegative.