Although the availability and quality of medicine continued to improve, epidemic diseases continued to spread, aided by modern transportation. An influenza pandemic, the Spanish Flu, killed 25 million between 1918 and 1919, while AIDS is yet uncured and treatments remain too expensive for wide use in developing countries.
The widespread use of petroleum in industry -- both as a chemical precursor to plastics and as a fuel for the automobile and airplane -- led to the vital geopolitical importance of petroleum resources. The Middle East, home to many of the world's oil deposits, became a center of geopolitical and military tension throughout the latter half of the century.
A vast increase in fossil fuel consumption leads to depletion of natural resources, while air pollution possibly leads to global warming and the ozone hole. The problem is increased by world-wide deforestation, also causing a loss of biodiversity. The problem of a depletion of natural resources is decreased by advances in drilling technology which led to a net increase in the amount of fossil fuel that is readily obtainable at the end of the century, as compared with the amount considered obtainable at the beginning of the century.