On May 17th, 1749, Edward Jenner, an English physician and scientist, was born in Berkeley, Gloucestershire, England. Jenner is widely regarded as the pioneer of the smallpox vaccine and is often referred to as the "father of immunology."

Jenner's groundbreaking work began when he noticed that milkmaids who had contracted cowpox, a milder disease related to smallpox, seemed to be immune to smallpox itself. This observation led him to hypothesize that deliberately infecting people with cowpox might protect them from the far deadlier smallpox virus.

In a famous experiment on May 14, 1796, Jenner inoculated an 8-year-old boy named James Phipps with pus from a cowpox lesion on the hand of a milkmaid named Sarah Nelmes. Later, when Jenner exposed Phipps to the smallpox virus, the boy did not contract the disease, proving Jenner's hypothesis correct.

This event marked the beginning of what would become known as vaccination, a term derived from the Latin word "vacca," meaning cow. Jenner's work laid the foundation for the development of vaccines against many other diseases, and his contributions to medical science have saved countless lives over the past two centuries.

Jenner's birthday, May 17th, is now celebrated as "World Immunization Week" by the World Health Organization to promote the use of vaccines and to raise awareness about the importance of immunization in protecting public health.