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For surveillance purposes, a confirmed case of HA is a clinically compatible illness with either: 1) a 4-fold change in antibody titer by IFA in acute and convalescent sera, 2) PCR amplification of HA DNA from a clinical sample, or 3) a smear that is positive for morulas in the granulocytes (or monocytes !) and an IFA titer of >1:64.