**Maurizio Iaccarino**

Institute of Genetics and Biophysics "A. Buzzati–Traverso", CNR, 111 Via Pietro Castellino, 80131 Naples, Italy; iaccarin@igb.cnr.it; Tel.: +39-0816132431

† This paper is an elaboration of the text of a lecture given at the International Congress "*Laudato si'y Grandes Ciudades*", which took place in Rio de Janeiro, Brazil, in July 2017.

Received: 19 December 2018; Accepted: 19 February 2019; Published: 22 February 2019

**Abstract:** Water, essential for the biology of living organisms, is also important for agriculture, for the organization of social life and for culture. In this review we discuss the interrelationship between water availability and human population size. The total population of the globe, 3–5 million people between the years 25,000 and 5000 Before Common Era (BCE), increased about 50-fold in coincidence with the development of agriculture. Later on, after the year 200 Common Era (CE), the number of people did not change appreciably and increased slowly in the period 1000 to 1500 CE. We show that the main cause of this observed slow-down in population growth was the increase in population density, which caused the appearance and spreading of infectious diseases, often due to the use of contaminated water. Population started to increase again when people learned how to use appropriate sanitation and hygienic rules. The managemen<sup>t</sup> of water resources, including transport of water to the areas where it is needed, separation and depuration of wastewater and production of freshwater by desalination, have become increasingly important. The population level is today very high and will continue to grow, thus causing a further increase in the density of people and an increased risk of contagious diseases. Therefore, more water for sanitation will be needed all over the world.

**Keywords:** freshwater needs; population density; epidemics; sanitation; global water requirements
