**Core Concept:**
Smallpox is a highly contagious viral disease caused by the variola virus. Vaccination with live attenuated smallpox vaccine (OPV) or killed smallpox vaccine (Dryvax) was formerly used to prevent smallpox infection. However, due to the eradication of smallpox in 1980, OPV is no longer used globally. Cowpox is a related alpha-poxvirus infection, which can provide cross-protection against variola virus upon exposure.

**Why the Correct Answer is Right:**
The correct answer, C (vaccination with cowpox), represents the concept of heterologous immunity or cross-protection. When a person is infected with or vaccinated with cowpox, their immune system mounts a response against cowpox proteins and antigens, which also recognizes and responds to variola virus proteins and antigens. This cross-reactivity provides some protection against subsequent smallpox infection.

**Why Each Wrong Option is Incorrect:**
A) Vaccination with live smallpox vaccine (OPV) is not the correct answer, as it is no longer used due to the eradication of smallpox and potential adverse reactions associated with the live virus.
B) Vaccination with killed smallpox vaccine (Dryvax) does not provide cross-protection against smallpox upon exposure to cowpox.
D) Infection with cowpox itself does not provide immunity against smallpox, as the immune response would be specific to cowpox rather than smallpox.

**Clinical Pearl:**
In the pre-eradication era, vaccination with cowpox (option C) could be considered as a method to protect against smallpox, especially in areas where smallpox was endemic. However, due to the eradication of smallpox, this practice is no longer advised. The concept of cross-protection from cowpox serves as a reminder of the interconnectedness of viral infections and the importance of vaccination programs to eliminate diseases.

**Correct Answer: C** (vaccination with cowpox)