Much of the progress achieved in recent decades improving human health is at risk. Severe economic and social crises, and natural disasters in diverse places have caused many national health systems to collapse, resulting in a resurgence of diseases that were once under control. Disabled by these diseases, some societies have been unable to recover. Conditions are critical where there is a proliferation of slums and squatter settlements with millions of people lack safe and adequate drinking-water, sanitation and solid-waste disposal facilities. Almost half the world's population suffers from diseases associated with insufficient or contaminated water where they are at risk from waterborne and foodborne diseases, of which diarrhoeal diseases are the most deadly.
Some 29 new diseases have emerged in the last 20 years. Other infections are now so resistant to drugs that they are virtually untreatable. Making matters worse, deadly new diseases such as AIDS and Ebola haemorrhagic fever, for which there is no cure or vaccine, are emerging in many parts of the world. Diseases that once seemed to be subdued, such as tuberculosis and malaria, are fighting back with renewed ferocity. Some diseases, such as cholera and yellow fever, are striking in regions once thought safe from them.
There are signs that diseases which emerged from Asia to ravage Europe in the past might be returning to Russia and, with the opening of borders, could spread westwards through Europe. In 1993 a woman is southern Kazakhstan was diagnosed with Bubonic Plague. Tuberculosis is soaring in Russia and malarial mosquitoes are breeding in the reservoirs around Moscow. For the first time in several years, two cases of cholera also occurred in Moscow.
However much society worldwide shows signs of fragmentation, expressed in the conventional names First, Second, Third and even Fourth World, their interdependence remains close. When this interdependence is separated from its ethical requirements, it has disastrous consequences for the weakest. Indeed, as a result of a sort of internal dynamic and under the impulse of mechanisms which can only be called perverse, this interdependence triggers negative effects even in the rich countries. It is precisely within these countries that one encounters, though on a lesser scale, the more specific manifestations of under development. Thus it should be obvious that development either becomes shared in common by every part of the world or it undergoes a process of regression even in zones marked by constant progress. This tells us a great deal about the nature of authentic development: either all the nations of the world participate, or it will not be true development. (Papal Encyclical, Sollicitudo Rei Socialis, 30 December 1987).