Back during the hunter gatherer times, the people were less exposed to rotting meat and night soil due to their constant moving so they weren’t as exposed to parasites. The tribes were also relatively small so they didn’t have the opportunity to pass on diseases to lots of people. They did however get intestinal worms, lice, fleas and they also sometimes encountered microbes from some of the wild animals they encountered. From 8000 BCE to 1750 CE the population grew 160 times to about 800 million. As the population increased they began to increase in agriculture which led to denser populations. People staying in one place meant that people could reproduce faster which required more food. This meant they had to improve the production of food and livestock. Because of this the humans and animals shared the same water supply and the conditions for the villages were a nesting site for rats, mice, ticks, flies, and mosquitoes. There were upsides to animal domestication such as food, material for clothing, fertilizer, land transportation and labor. But animal domestication also is a popular source for human diseases. The major diseases that ended up killing a lot of people came from evolving animal diseases. As humans expanded they also cleared out forests which killed animals and allowed for the spread of insects and rodents.

If you look at dates of the most common infectious diseases today, they are all relatively recent. The forests are continually being cleared out for resources and farmland which continues the disease problem. The ditches and irrigated fields serve as breeding ground for insects and other parasite harvesting organisms. The consumption of bush meat has also been a big issue when it comes to the spread of disease due to the availability of the meat and the disease potential the meat has. The direct movement of people into habitats with bush mean may have contributed to the emergence of Ebola and Aids. The spread of these diseases over wide areas is a modern day issue due to how easy it is for someone to fly from one country to the next. Modern farming practices in association with trade travel, and ecological change are responsible for the emergence of some diseases such as mad cow disease, foot in mouth disease, and Nipah virus.

As the population has been increasing our climate change potential has also been increasing more people means more strain of the environment such as soil exhaustion, water depletion and the loss of various species of wild animals and plants. The primary cause of climate change is due to global warming because of greenhouse gas emissions from cars and factories. This process of global warming is also enhanced by the negative feedback loop escalating the rate at which this is occurring. The primary risk to health due to climate change is the direct biological consequence of heat waves, extreme weather events, and temperature enhancement due to air pollutants. The secondary risks affect the amount of food, water flow, infectious disease vectors, and immediate host ecology for zoonotic diseases. The tertiary risks include the consequences due to the decline of basic resources.