According to statistics from the World Health Organization and UNICEF, 2.6 billion people worldwide engage in open defecation.
The lack of toilets is the cause of 2 million preventable deaths a year, mostly from a variety of dysentery-like ailments that result from ingesting human fecal matter.
Without toilets, it's extremely difficult to avoid ingesting human waste. Either it enters the food and water supplies, or it is spread by flies and dust.
The Los Angeles Times reports:
"When people don't see the problem, they don't push for solutions. That means there's little incentive for governments to provide sewer systems and septic tanks to city dwellers, or for people in the country to dig and maintain outhouses."