According to UN-Habitat, around 1.1 billion urban residents live in slums or informal settlements: makeshift housing with no formal tenure, often without running water or sanitation. That is roughly one city dweller in four. The mechanism: in many cities, population grows faster than housing and infrastructure can be built, and informal housing fills the gap.
That's the estimate from a WHO-led study published in The Lancet in 2024, covering the half-century since the Expanded Programme on Immunization was launched in 1974. It works out to about six lives saved every minute, mostly infants. The measles vaccine alone accounts for around 60% of those lives.
According to the World Gold Council, about 216,000 tonnes of gold have been mined in all of human history. Because gold is so dense (19.3 tonnes per cubic metre), it would fit in a cube about 22 metres on each side, roughly the height of a seven-storey building. Strikingly, about two-thirds of it has been mined since 1950.