Vaccines and the history behind their discovery

Infectious diseases have long shaped human history, from the 14th century Black Death and the 1918–19 influenza pandemic to today’s emergence of the COVID-19. As new vaccines are getting ready to be administered against this latest pandemic, what’s the long history behind their discovery? And why are they still hotly debated despite their many global public health successes in eradicating deadly diseases like smallpox and polio.