For centuries, well before the basics of infectious disease were understood, humans have realized that climate change influences epidemic disease (Patz et al.). Roman aristocracy retreated to the hills each summer to avoid malaria, and South Asians learned that in early summer, heavily spiced foods were less likely to cause diarrheal disease (Patz et al.). Patz et al. stated that there were three distinct transition periods that changed the relationship between humans and microbes. These three transition periods are: 1) early human settlements that allowed enzootic infectious species to enter the human population, 2) early civilizations in Eurasia exchanged dominant infections via military and commercial contacts, and finally, 3) European expansionism over the last five centuries has caused the spread of often lethal infectious diseases. They also argue that we may be in the fourth transition, in which climate change will have a wide range of impacts on the emergence of infectious diseases in human populations. Most climate scientists agree that the primary cause of global climate change is human expansion of the greenhouse effect. effect. This is global warming that occurs when the atmosphere traps heat radiating from Earth into space (climate.nasa.gov). Major gases that contribute to the greenhouse effect include water vapor, carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (climate.nasa.gov). Human activities are modifying the natural greenhouse effect. The burning of fossil fuels such as coal and oil has increased the amount of carbon dioxide in the atmosphere (climate.nasa.gov). Carbon dioxide levels have increased from 280 parts per million to 379 parts per million over the past 150 years due to human action... middle of paper... conditions more conducive to disease in regions that previously did not harbor disease or carriers of diseases (climate.org). Climate change accelerates the spread of disease largely because warmer temperatures increase the geographic range in which disease-carrying animals and insects can survive (climate.org). Climate change contributes to the increased occurrence of extreme events such as floods, storms, droughts and uncontrolled fires which can also have a negative effect on human health (climate.org). Floods can spread bacteria and viruses and can also contribute to the reproduction of insects (climate.org). All of these things together will have huge implications on human health. We might expect to see a higher infection rate and a higher death rate in some areas. We might also expect children and the elderly to get sick and possibly die even more often.