Kinda' hard to decide because there are so many issues. Unfortunately there are too many that are interrelated. What comes to mind is 'safe drinking water'. Perhaps that comes from the TV adds today.
With research the CDC says overall it affects 3 continents - Africa, Asia, and South America. In other words they recommend not drinking natural and tap water if traveling. Those not affected are North America, Oceania, and Antarctica. With Europe they say the EU countries are okay, but Russia is not. The unsafe continents account for 5 billion people while safe is 1.1 billion (2013). So, it appears to be a problem . . .
But, we know here in the U.S. safe drinking water today is an issue . . . look at the current trend with bottled drinking water and not trusting tap water. Our water is being polluted both man-made and natural. To compound that "40 of 50 states expect water shortages in some portion of their states under average conditions in the next 10 years" (EPA).
How to fix?