After needing 13 liters of blood for a surgery at the age of 13, a man named James Harrison pledged to donate blood once he turned 18. It was discovered that his blood contained a rare antigen which cured Rhesus disease. He has donated blood a record 1,000 times and saved 2,000,000 lives.
Maurice Hilleman developed vaccines for measles, mumps, hepatitis A, hepatitis B, chickenpox, meningitis, pneumonia and Haemophilus influenzae, among many others. He saved more lives than any other scientist during the 20th century.