From William MacAskill's Doing Good Better.
In the twentieth century alone, smallpox killed more than three hundred million people. Fortunately, in 1977, we eradicated it.
It's difficult to comprehend just how great an achievement this was, so let's make a comparison. Supposed we'd achieved world peace in 1973. How many deaths would have been prevented? That timescale includes the killings of Cambodia's Khmer Rouge, the Rwandan genocide, the two Congo wars, the 9/11 attacks and the wars in Afghanistan and Iraq. If you add up all the wards, genocides, and terrorist acts that occurred since 1973, the death toll is a staggering twelve million people. Prior to its eradication, smallpox killed 1.5 to 3 million people every year, so by preventing these deaths for over forty years, its eradication has effectively saved somewhere between 60 and 120 million lives. The eradication of smallpox is one success story from aid, saving five times as many lives as world peace would have done.