Explanation:

The correct answer is option 'A' - Since smallpox virus is closely related to cowpox virus.

Background:
Smallpox and cowpox are two viral diseases that affect humans. Smallpox is caused by the Variola virus, while cowpox is caused by the Cowpox virus. Both diseases can cause skin lesions and have similar symptoms, but smallpox is much more severe and often fatal.

Observation:
During the 18th century, it was observed that milkmaids who had contracted cowpox, a relatively mild disease, did not suffer from smallpox later. This observation led to the development of the smallpox vaccine by Edward Jenner, who used cowpox virus to protect against smallpox.

Explanation:
The reason behind this observation lies in the similarity between the two viruses. Smallpox virus and cowpox virus are closely related and belong to the same family of viruses called the Orthopoxviridae. Although the viruses are distinct and cause different diseases, they share enough similarities for the immune system to recognize and mount a response against both.

When a person is infected with cowpox, their immune system generates an immune response to fight off the virus. This immune response not only clears the cowpox infection but also creates memory cells that recognize and remember the virus. These memory cells can recognize similar viruses, including smallpox, due to the shared antigens and structures between the two viruses.

Immune response:
When a person with previous cowpox infection is exposed to smallpox, their immune system recognizes the smallpox virus as a threat. The memory cells produced during the cowpox infection quickly mount an immune response, preventing the smallpox virus from causing a severe infection. As a result, the person does not suffer from smallpox or experiences a much milder form of the disease.

Significance:
This observation and understanding of the immune response led to the development of the smallpox vaccine. Edward Jenner realized that by infecting individuals with the cowpox virus, he could protect them from smallpox. This discovery revolutionized the field of vaccination and paved the way for the eradication of smallpox, making it the first disease to be eradicated globally.

Overall, the close relationship between the smallpox and cowpox viruses, and the resulting immune response, explains why milkmaids who had cowpox did not suffer from smallpox later. This understanding formed the foundation for the development of the smallpox vaccine and the eventual eradication of smallpox.