The world is rapidly changing, complex and interconnected human, animal and environmental health affect on each other. This is an ecological approach to health on collaborative, integrated, and multidisciplinary. The effects of social and cultural changes on infectious diseases have been observed in populations. An example would be the 2016 Anthrax outbreak in Siberia. A heat wave led to unfreezing of a reindeer carcass that had been frozen in permafrost; carcass had spores of Bacillus anthracis. One of the cases was a 12 year old boy who died of intestinal anthrax. Hunter gatherers roamed over distances to find plants and find water. Population groups were small so diseases of crowds requiring human-to-human transmission were absent. They experienced many infections with intestinal worms, lice and fleas had adapted to humans. They sometimes encountered microbes of wild animals and illnesses would have made isolated appearances but seldom, if ever, affected the entire group.
From 8000 BCE to 1750 CE, population of the world increased 160 times to 800 million. Growth spurt coincided with initiation of agriculture and domestication of animals (8000 BCE). The agricultural revolution had a big increase in agriculture and domestication of animals. They can support denser population in which people stayed in one place and they could grow food, birth spacing shortened more children and more people required more food. Animals frequently lived inside homes. The agricultural revolution had some negative effects since humans and animals lived together shared the same water supply. Squalid conditions of villages offered a paradise for rats, mice, ticks, flies, and mosquitoes. The human diet deteriorated due to concentration on growing and eating a single crop (lack of nutrients). The benefits of animal domestication were the hunter gatherers used food as meat and milk, clothing and fiber from wool and hides, manure for fertilizer, animal power for land transport of goods and people and for plowing. The downside of animal domestication was the source of human disease, waste accumulated as humans settled down. Animal bones, garbage, and feces attracted insect vectors of disease as well as wild birds and rodents carrying their own potential pathogens.
Major killers of humanity evolved from diseases of animals such as Eg, smallpox, influenza, TB, malaria, plague, measles, cholera. These killers adapted to humans (human to human transmission) and were referred to as “crowd diseases and need dense populations in order to facilitate transmission. Clearing of forests for planting of crops and destruction of game animals permitted the creation of new ecological niches for insects and scavenging rodents. Ditches, irrigated fields, and pottery vessels served as breeding grounds for insects and snails. Agricultural civilizations appeared in the riparian river environments such as the Tigris and Euphrates valley along with the Nile and Yellow Rivers where irrigation farming could be practiced. Irrigation was beneficial for drowning weeds that would compete with crops.
The roleobalization played a big role in ancient history. By Roman times, populations of Europe, Asia, and North Africa were joined into one giant breeding ground for microbes. In 165-180 CE the Plague of Antonine reached Rome, killing millions of people. In 541 CE and until 757 CE the Plague of Justinian began in Constantinople from the 6th and 8th centuries. Researchers found tiny bits of DNA in the teeth of two German victims killed by the Justinian plague. Malaria occurred in three countries such as Rome, England, and the Americas. Because these diseases killed European workers on tobacco, rice, and sugar plantations, colonists imported labor in the form of African slaves who had been exposed to smallpox and malaria were less vulnerable to diseases. Yellow fever was carried by African wild monkeys.