Description: | Caption: This 1962 photomicrograph depicts a chorioallantoic membrane (CAM) that had been infected with variola, and subsequently stained with rabbit anti-vaccinia conjugate, coupled with fluorescein isothiocyanate (FITC), in a smallpox study. Positive control smears prepared from vaccinia- and variola-infected CAM, revealed numerous fluorescing particles. Likewise, fluorescence was absent in the inhibited smear, and in the smear stained with normal rabbit globulin conjugate. No fluorescence was observed in the negative controls. |