Jenner prepared the vaccine of smallpox by using pathogens from Cowpox pustule.

Explanation:
Cowpox is a viral disease that affects cows and sometimes other mammals. It causes skin lesions or pustules, similar to those seen in smallpox. Jenner observed that milkmaids who had contracted cowpox did not seem to get smallpox. This observation led him to hypothesize that exposure to cowpox protected against smallpox.

To test his hypothesis, Jenner collected material from a cowpox pustule on a milkmaid named Sarah Nelmes. He then transferred this material to the arm of a young boy named James Phipps. Phipps subsequently developed a mild case of cowpox.

After a few weeks, Jenner exposed Phipps to smallpox by inoculating him with material from a smallpox pustule. Surprisingly, Phipps did not develop smallpox. Jenner repeated this experiment with several other individuals and observed the same results.

Based on these findings, Jenner concluded that exposure to cowpox protected against smallpox. He called this process vaccination, derived from the Latin word "vacca" meaning cow. Jenner's method involved using pathogens from cowpox pustules to induce immunity against smallpox.

The use of cowpox as a vaccine was significant because it provided a safer alternative to the traditional method of inoculation, which involved directly infecting individuals with smallpox. Cowpox caused a milder disease in humans, and individuals who were vaccinated with cowpox were subsequently protected against smallpox.

Jenner's discovery paved the way for the development of vaccines and the field of immunology. His work demonstrated the principle of using weakened or killed pathogens to stimulate an immune response and provide protection against infectious diseases. Today, vaccines are crucial in preventing and controlling numerous diseases worldwide.

In conclusion, Jenner prepared the smallpox vaccine by using pathogens from cowpox pustules. This discovery revolutionized medicine and led to the development of vaccines, ultimately saving countless lives from the devastating effects of smallpox.