COVID-19 is a disease with no proven pharmaceutical intervention and no proven vaccine. In such circumstances, prevention is all we have. The role of handwashing in the prevention of communicable diseases has been known for over a century, yet it remains severely neglected as a public health investment, to be periodically re-discovered during pandemic-scale infections. Over 26% of the global population has no access to a handwashing station in the home; for many low-income countries this proportion rises to over 50%. In other instances, the water is unaffordable or the supply has been shut off on account of unpaid bills. But when there is no water in the home or yard, or no mechanism for delivering enough water, good hand-washing is extremely difficult. Well before COVID-19, global cost-benefit analyses of water and sanitation investments, with benefits measured in time-savings as well as health, showed significant net benefits in all sub-regions of the developing world. This Viewpoint paper argues that, in the current crisis and its aftermath, it is imperative for governments and donors to prioritize and generously fund affordable, reliable, and accessible water services in underserved regions of the world. More than ever before, this is a foundational investment for health, dignity and development.