Soldiers tend to be young adults, generally in the third to fourth generations of life. Acceptance to military service almost always entails medical screening, so that soldiers tend to be healthy individuals with little chronic disease. Soldiers around the world train with various types of weaponry and heavy machinery. Some may live under field conditions without plumbing or running water. Troops are deployed at times to areas far from home, and may be exposed to pathogens not found in their native lands. Even when stationed at their home base, personnel living in barracks are often subjected to relatively crowded living conditions which may facilitate the transmission of infectious diseases.