Academic discussions of the main factors driving improvements in longevity over time abound. Probably dozens of papers have been written on this topic. There has been intense and sometimes entertaining debate. John Snow and his contemporaries debated miasma and microbes as causes of ill health in 19^th century Britain. Thomas McKeown argued for nutritional improvements as opposed to improved access to antibiotics as a key determinant of the increase in life expectancy over the 19th and 20th centuries. Today there is much debate on the driver of recent increases in mortality observed in the United States. Is it drugs, alcohol, suicide, obesity-related diseases, or underlying despair?

Despite these debates, few would argue with the critical roles of two simple and yet powerful factors in improving population health over the last two centuries: vaccination and providing access to clean water. They are both true population-level interventions. Their impact is contingent on their application across the population as a whole. And yet they are both quite different in their approach to achieving population impact.

Vaccination is by definition a biomedical intervention: it involves inoculating individuals with an attenuated or otherwise transformed agent that generates an immune response. The intervention is applied at the individual-level: literally person by person, one by one. But the beauty of its effect is that, when a certain number are immunized, even those who were not vaccinated are protected. This phenomenon is referred to as herd immunity. It explains why even children who cannot receive the vaccine because of reduced immunity are protected when they are in groups in which at least 95 percent have been vaccinated. In an elegant example of the critical role of context, an attribute of the group protects the individual.

Access to clean water is a very different kind of intervention. It is an engineering intervention that affects everyone at the same time. It is not possible to opt out of it. It requires building infrastructure to obtain, clean and distribute safe drinking water, water that is free of microbes or other dangerous contaminants. It is for everyone, regardless of class, race or origin. It is a perfect example of how planning and technology can make real a simple organizing principle that everyone should be able to open their faucet and drink straight out of it.

And yet, despite the obvious benefits of vaccinations and clean water we live in a society where both of these basic human rights are challenged every day. Measles is surging all over the world in a clear example of the failure of institutions to ensure an adequate vaccine coverage using a well-established, safe and exceptionally effective vaccine. Failures of the system, misinformation, and distrust of science and medicine results in large pockets of unvaccinated children.

Access to clean water remains a challenge all over the world from Nairobi, Kenya, to Lima, Peru to Flint, Michigan. Despite our engineering prowess we fail to deliver a resource fundamental to life, as fundamental as oxygen, to a substantial proportion of the world's population. In a dramatic illustration of the failures of our system to protect the most basic rights of humanity, access to Coca Cola, Pepsi and other sugar sweetened beverages (with all this implies for health) is often easier than access to clean safe water. We advertise, promote and sell these products all over the world even in the remotest locations, even when basic access to clean water is far from guaranteed.

It is gratifying that we as a society have invented vaccines to protect ourselves from life-threatening infections and can build sophisticated engineering systems that allow us to deliver safe water to millions. Both population interventions have done and can do so much more to promote global health and well-being. But we are far from making their population impact a reality. Our job now is to make sure that these basic human rights, the right to be vaccinated and protected from infectious diseases and the right to open the tap and drink safe water, are extended to everyone.

Ana V. Diez Roux, MD, PhD, MPH
Dean and Distinguished Professor, Epidemiology, Dornsife School of Public Health