A 35-year-old male first-time blood donor who donated in June 2011 tested positive for HBV (hepatitis B virus) DNA (Box). DNA sequencing analysis identified HBV genotype E. The only other HBV marker present was anti-HBs (antibodies to HBV surface antigen), attributable to a full course of vaccination in 2003-2004. A followup interview identified heterosexual contact with a new partner as the likely source of infection. HBsAg (HBV surface antigen) remained undetectable throughout the 90-day follow-up period, and the donor was asymptomatic. Viral load peaked at 2171U/mL and DNA was undetectable by 3 months. Coincident with DNA disappearance, the anti-HBs concentration increased from 1011U/L to 5181U/L, with subsequent detection of IgM anti-HBc (1gM antibodies to hepatitis B core antigen) and total anti-HBc indicative of recent infection.