In the temperate climate zones, many familiar viral and bacterial diseases are spread directly from person to person, by airborne routes of transmission or by sexual contact. In the tropics, respiratory diseases(such as measles, respiratory syncytial virus, tuberculosis) and sexually transmitted diseases are also of great importance. In addition, many diseases are spread by contaminated water and food sources, since clean water and sanitary conditions are often a luxury in developing countries. Alternatively, some tropical disease agents are transmitted by an intermediate carrier or vector. The insect or other invertebrate vector picks up the pathogen from an infected person or animal and transmits it to others in the process of feeding. Often, tropical disease agents must undergo important developmental changes within the vector before they complete their life cycle and once again become infective for man. Infectious diseases account for approximately one half of all deaths in the tropical areas of the world. Nearly all of these deaths occur in children under the age of five. Those infectious diseases that are primarily found in the developing world or profoundly impact the health of people living in the tropics are frequently referred to as tropical diseases. Conditions that contribute to the risk for becoming infected with a tropical disease agent include biological factors related to population density, rural vs. urban living, nutritional status, climate and other environmental factors, as well as socioeconomic circumstances. At one time, many of today's tropical diseases also occurred in temperate regions, since many of the same risk factors were found there. Today, however, industrial development and technological advances, including medical ones, have lessened the impact of these diseases in industrialized countries.