**Core Concept**
Smallpox is a viral disease caused by the variola virus, which belongs to the Orthopoxvirus genus. The disease has been a significant public health threat for centuries, characterized by fever, headache, and a characteristic rash that progresses to form pus-filled lesions.

**Why the Correct Answer is Right**
The eradication of smallpox was achieved through a global vaccination campaign led by the World Health Organization (WHO). The last naturally occurring case of smallpox was reported in Somalia in 1977. The virus was declared eradicated in 1980, making smallpox the first and only human disease to be completely eliminated from the planet. The success of the eradication campaign was largely due to the development of a highly effective vaccine by Edward Jenner in 1796.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as there is no conclusive evidence to support the eradication of malaria.
**Option B:** This option is incorrect as polio has not been eradicated, although significant progress has been made in reducing its incidence.
**Option C:** This option is incorrect as HIV/AIDS is still a major public health threat worldwide, with no cure or vaccine available.

**Clinical Pearl / High-Yield Fact**
The eradication of smallpox serves as a powerful example of the impact of global cooperation and vaccination efforts on public health. It highlights the importance of continued investment in vaccine development and disease surveillance to prevent and eliminate infectious diseases.

**Correct Answer: D. Smallpox.**