A virus is the causative agent of:

The correct identification of a virus as the causative agent is exemplified by smallpox, which is caused by the variola virus. This virus specifically targets the human population, leading to severe symptoms and widespread outbreaks before its eradication through vaccination efforts. The historical impact of smallpox highlights the role of viruses in public health, as they can cause significant morbidity and mortality.

In contrast, influenza is also caused by a virus (influenza virus), indicating that both smallpox and influenza are associated with viral infections. However, in this context, the emphasis is specifically on smallpox, which is often more prominently recognized in discussions of viral diseases and their eradication.

Strep throat, on the other hand, is caused by bacteria (Streptococcus species), and malaria is a parasitic disease caused by Plasmodium species. These distinctions underline the importance of correctly identifying pathogens when discussing diseases in public health, as the treatment and prevention strategies differ between viral, bacterial, and parasitic infections. Thus, understanding the specific causative agent is crucial for effective public health strategy formulation.