Clean water and&#x2F;or sewage. Almost a billion people don&#x27;t have access to either, because the infrastructure doesn&#x27;t exist for municipal sewage or water treatment. Huge numbers of people die from preventable diseases, simply because their water is fouled.<p>Until cellphones, most of the world didn&#x27;t have reliable communications, because nobody could build the necessary infrastructure. Wireless changed that. We need the equivalent breakthrough for water treatment.<p>Dean Kamen is doing interesting stuff in this area (a stirling-engine based system for water distillation [1]), but his approach is still limited by costs and distribution.<p>Solving this problem would literally change the world, and unlike many of the suggestions here, it&#x27;s an area where there&#x27;s hope that a small research team could make a dent (for example, Kamen wanted to raise $1M for Slingshot.)<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Slingshot_(water_vapor_distillation_system)" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Slingshot_(water_vapor_distill...</a>