A field hospital is a temporary hospital or mobile medical unit that takes care of casualties on-site before they can be safely transported to more permanent facilities. This term was initially used in military medicine (such as the Mobile Army Surgical Hospital or MASH), but it is inherited to be used in disasters and other emergency situations. 
A field hospital is a medical staff with a mobile medical kit and, often, a wide tent-like shelter (at times an inflatable structure in modern usage) so that it can be readily set up near the source of casualties. In an urban environment, the field hospital is often established in an easily accessible and highly visible building (such as restaurants, schools, hotels and so on). In the case of an airborne structure, the mobile medical kit is often placed in a normalized container; the container itself is then used as shelter. A field hospital is generally larger than a temporary aid station but smaller than a permanent military hospital.
International humanitarian law such as the Geneva Conventions include prohibitions on attacking doctors, ambulances, hospital ships, or field hospitals buildings displaying a Red Cross, a Red Crescent or other emblem related to the International Red Cross and Red Crescent Movement; deliberately attacking or otherwise causing harm on these health facilities (especially during warfare or armed conflicts) may constitute a war crime.
Field hospitals are also prevalent in the event of disease outbreaks and pandemics. The ongoing COVID-19 pandemic led to the establishment of field hospitals in many parts of the world, especially in the developing world.
The PMA is organized in four zones:
In case of really massive disaster, it is possible to have several PMA; the evacuation goes then not directly to a hospital, but to another big field hospital called "medical evacuation centre" (centre médical d'évacuation, CME), to avoid the saturation of the hospitals.
In case of a red plan, the PMA is under the responsibility of a physician chosen by the director of medical rescue (DSM), and he is assisted by a firefighter officer chosen by the commander of rescue operation (COS). The firefighter officer has in charge the identification of the living casualties and of the secretaryship. The aim of the PMA is to sort and stabilize the casualties before their evacuation to a hospital.
A similar system can be set up as a preventive measure for some very big events (sport championship, cultural events, concert...), but managed by first aid associations. It is then called an "associative medical post" (poste associatif médicalisé, PAM). (For smaller events, is simple first aid post, with only volunteer certified first responders and no medical staff, is set up.)
The civil defence military units (Unité d'instruction et d'intervention de la sécurité civile, UIISC) have airborne field hospitals. The general system is called DICA (détachement d'intervention de catastrophe aéroporté, i.e. airborne disaster unit), and is specialized in search-and-rescue and in emergency medicine; it can be enhanced by the Fast civil defence medical unit, called ESCRIM (élément de sécurité civile rapide d'intervention médicale). The ESCRIM is a surgical unit (detachement d'appui chirurgical) assisted by a medical assistance unit (DAMHo, détachement d'appui médical et d'hospitalisation); the later is specialized in pre- and post-operation care, and allows 48h of hospitalization. The UIISC also has a PMA (i.e. sort, stabilization and evacuation structure) when the hospital infrastructure of the country is sufficient.
The Namibian Defence Force operates a mobile field hospital through its Defence Health Services Directorate. It was donated by the German government to Namibia in March 2013. Initially it was a UN level two hospital but has now been upgraded to level one. The field hospital is containerized in tents, it has capacity to treat forty outpatients per day and can admission capacity of twenty patients. It has two intensive care units, laboratories, an X-ray unit and a mobile oxygen concentrator. The dental department can treat 20 and four operations can be carried out daily. It has its own mobile logistics support wing consisting of kitchens, water purifiers, water tanks, toilet and shower containers, generators and sewage and refuse disposal facilities.
In the United States Army Medical Department, the term "field hospital" is used as a generic term for a deployable medical facility. However, from 1906 to the present, with small interruptions, the department has had specific organized units called "Field Hospitals." These numbered units, for example the 10th Field Hospital have specific tables of organization and equipment, capabilities, and doctrine for their employment, all of which have varied over time. Readers should take care not to confuse the generic American field hospital with a specific numbered XXth Field Hospital, as the two cannot be used interchangeably. An Evacuation Hospital, a Mobile Army Surgical Hospital (MASH), a Combat support hospital (CSH), a Field Hospital and a numbered General Hospital are all field hospitals—but a MASH, CSH, a General Hospital or a Field Hospital are not interchangeable.
Field hospitals were mobile, were sent to support the battle line—as was the 103d—and served in every capacity, from disease hospital to resuscitation center to acute care (with surgical reinforcement) to reserve and rest status. Their job was triage, stabilization, and evacuation to the base hospitals.
Media related to Field hospitals at Wikimedia Commons