Accurate diagnosis of smallpox relied heavily on meticulous clinical examination, particularly of the rash's distinctive evolution. The diagnostic process focused on three key elements: timing, distribution, and lesion characteristics. Physicians looked for the tell-tale progression from macules to papules, vesicles, pustules, and finally scabs, with all lesions on any body part developing at the same stage—a crucial difference from chickenpox, where lesions appeared in successive crops. The rash also showed a "centrifugal distribution," concentrating more heavily on the face, hands, and feet rather than the torso. This contrasted sharply with chickenpox's "centripetal" pattern (more on torso). For uncertain cases, laboratory confirmation using electron microscopy to identify the brick-shaped variola virus or growing the virus on chorioallantoic membrane of chick embryos provided definitive diagnosis. These methodical observation protocols, later supplemented by WHO's diagnostic cards distributed to field workers, were essential for correctly identifying cases and implementing effective containment measures like ring vaccination.