According to Wikipedia, smallpox 'killed an estimated 400,000 Europeans annually during the closing years of the 18th century (including five reigning monarchs)

According to Wikipedia, smallpox 'killed an estimated 400,000 Europeans annually during the closing years of the 18th century (including five reigning monarchs), and was responsible for a third of all blindness. Of all those infected, 20–60%—and over 80% of infected children—died from the disease. Smallpox was responsible for an estimated 300–500 million deaths during the 20th century.' Of course, this disease was contained by vaccines. And immunology is definitely more broad than mere vaccines. In fact the modern immunology plays a quite central role in today's research of physiology, virology, microbiology and pathology. Back in 2003, when SARS broke out in Beijing, the streets were emptied. Nobody wanted to meet each other, nobody dared. If immunology had not existed, such scenario might not be just one incidence, but incidence after incidence.