The construction of new villages and housing developments in rural areas force animals to live in dense populations, creating opportunities for microbes to mutate and emerge. Changes in agriculture. The introduction of new crops attracts new crop pests and the microbes they carry to farming communities, exposing people to unfamiliar diseases. The destruction of rain forests. As countries make use of their rain forests, by building roads through forests and clearing areas for settlement or commercial ventures, people encounter insects and other animals harboring previously unknown microorganisms. Uncontrolled urbanization. The rapid growth of cities in many developing countries tends to concentrate large numbers of people into crowded areas with poor sanitation. These conditions foster transmission of contagious diseases. Modern transport. Ships and other cargo carriers often harbor unintended "passengers", that can spread diseases to faraway destinations. While with international jet-airplane travel, people infected with a disease can carry it to distant lands, or home to their families, before their first symptoms appear. Germ theory of disease
In Antiquity, the Greek historian Thucydides (c. 460 – c. 400 BCE) was the first person to write, in his account of the plague of Athens, that diseases could spread from an infected person to others. In his On the Different Types of Fever (c. 175 AD), the Greco-Roman physician Galen speculated that plagues were spread by "certain seeds of plague", which were present in the air.
In the case of scuba and surface supplied divers, the diver breathes gas at ambient pressure, and if this is not able to escape freely during ascent, the pressure difference will cause it to expand, and may rupture the lung tissues (pulmonary barotrauma), and escape to a variety of places, one of which can be the mediastinum. A diver with symptoms of mediastinal emphysema may also have any combination of arterial gas embolism, pneumothorax and subcutaneous or pulmonary interstitial emphysema. Factors which may prevent free escape of the compressed breathing gas include holding the breath or respiratory obstructions such as cysts, mucus plugs, or scar tissue.In rare cases, pneumomediastinum may also arise as a result of blunt chest trauma (e.g. car accidents, fights, over pressure of breathing apparatus), while still evolving in the same fashion as the spontaneous form.Pneumomediastinum is most commonly seen in otherwise healthy young male patients and may not be prefaced by a relevant medical history of similar ailments. Diagnosis
Pneumomediastinum is uncommon and occurs when air leaks into the mediastinum. The diagnosis can be confirmed via chest X-ray showing a radiolucent outline around the heart and mediastinum or via CT scanning of the thorax. Treatment
The tissues in the mediastinum will slowly resorb the air in the cavity so most pneumomediastinums are treated conservatively. Breathing high flow oxygen will increase the absorption of the air. If the air is under pressure and compressing the heart, a needle may be inserted into the cavity, releasing the air.