Vaccination. Training the army before the war.

A vaccine gives the immune system a weakened or inactive version of a threat. The body rehearses the fight without getting sick. When the real pathogen arrives, the immune system already knows the moves. The illness never gets a foothold. Vaccines do not treat disease. They prevent it from starting.

• Smallpox killed about 300 million people in the 20th century. Vaccination wiped it out by 1980.
• India's universal immunisation programme covers 12 diseases. The schedule starts at birth.