Vaccination with cowpox virus provides immunity against which disease?

Vaccination with the cowpox virus is historically significant because it is the basis for the smallpox vaccine. The cowpox virus, which is a milder virus, was used by Edward Jenner in the late 18th century to provide immunity against smallpox, also known as variola. The principle underlying this vaccination is that exposure to a less harmful pathogen (cowpox) can stimulate the immune system to recognize and fight off a related, more dangerous pathogen (variola). This foundational concept led to the eventual eradication of smallpox through vaccination efforts worldwide.

In contrast, the other diseases listed—varicella (chickenpox), measles, and rubella—are caused by different viruses and do not have any direct immunity relationship with the cowpox virus. Each of these diseases is prevented by its own distinct vaccines that target the specific viruses responsible for those illnesses. Thus, the cowpox vaccine specifically provides immunity against variola, making it the correct answer.