The field of ecological epidemiology studies the relationships among infectious pathogens, their hosts, and other environmental factors that influence the spread of the pathogens. Given the complex interdependencies of microbes and their environments, environmental factors must be considered when trying to understand any disease process. For example, cholera, an acute infection that causes severe diarrhea and can be fatal, is caused by the bacterium Vibrio cholerae, and people become infected by ingesting food or water that is contaminated with the bacterium. Cholera is rare in the United States where advanced sanitation systems and water treatment facilities provide easy access to fresh, safe water. However, in areas that lack clean water and basic sanitation—usually areas of low socioeconomic development—the disease persists and is easily spread. Legionellosis, or Legionnaires' disease, a type of pneumonia caused by the bacterium Legionella pneumophila, is often associated with contaminated, human-made systems affecting circulating air or water in more affluent communities. Such systems include hotel air conditioning units, industrial plumbing, decorative fountains, and hot tubs.
Many of the most consequential diseases are spread to humans and other animals by insect or arthropod vectors. For example, chikungunya, malaria, yellow fever, and West Nile virus are all diseases spread by mosquito vectors. These diseases can only exist where their vectors exist. For example, West Nile virus is found in birds but is not typically passed directly between birds. Instead, mosquitoes—the vectors—transmit the virus between birds and from birds to humans. Trying to control the disease often results in trying to control the vector. Many municipalities have implemented mosquito abatement programs that target mosquitoes directly with larvicides to kill the larvae and adulticides to kill adult mosquitoes. These programs almost always are accompanied by public education campaigns to try to diminish prime breeding conditions, such as standing water.
The reemergence and spread of the Zika virus by a mosquito vector is a useful case study for understanding diseases in the context of their ecology. Zika virus was discovered in the Zika Forest in Uganda, and cases have been sporadically detected throughout the tropics of Africa, Asia, and the Pacific. In 2007 the virus was found on islands much farther east in the Pacific Ocean. In 2015 it reached South America and triggered an extensive outbreak. In regions where Zika virus was originally found, it moved between mosquitoes and monkeys but rarely infected humans. Upon reaching South America, mosquito-to-human transmission became extensive.
The main vector for Zika virus is the mosquito Aedes aegypti, also known as the yellow fever mosquito because of its role in spreading that disease. This mosquito is widely distributed around the world, including in many large metropolitan areas, increasing the opportunities for the virus to spread from vector to human.Climate change exacerbates the potential human toll of Zika virus and related diseases by increasing the range of Aedes aegypti, which historically was restricted to tropical regions. As the world warms, the northern and southern extents of the Aedes aegypti range are expanding poleward. If this range expansion continues, many new potential hosts for Zika and related viruses will become susceptible. Finding adequate control measures to slow the spread and transmission of Zika and other mosquito-borne viruses requires knowledge of both the virus and its vector. Understanding and altering the environment can control vector populations and, therefore, affect the transmission of these viruses. The One Health Initiative is a multidisciplinary approach to optimizing the health of humans, other animals, and the environment at large. Humans remain dependent on the environment to supply food, water, and shelter. Human, animal, and environmental health are inextricably linked to one another. Pollution released by humans into the environment harms not only the ecosystem where it is released, but also all people and other animals that live there. When mercury is released from coal-burning power plants, for example, it can settle in the oceans, become incorporated into food chains by microbes, and end up in the flesh of fish people may eat.
The vast majority of communicable human diseases involve animals, either as reservoirs in which the disease resides and multiplies (e.g., Ebola) or as vectors, or transmitters, of the disease (e.g., malaria). With larger human populations, the probability of coming into contact with other animals increases, causing a higher likelihood of contact with reservoirs or vectors. More people means further expansion into natural areas that harbor infectious agents that can transfer to animals and people. Likewise, the ease of global mobility in modern societies allows for the rapid spread of introduced bacterial and viral pathogens around the world.Contamination of food by pathogens is a constant public health concern. Due to their regular contact with farmers and with meat-eating food consumers, domesticated animals used for meat can provide a reservoir for spreading pathogens to people. Crops are occasionally contaminated with animal or human sewage, with the potential to spread infections widely, such as in the 2018 outbreak of pathogenic E. coli contamination of precut romaine lettuce that resulted in 96 hospitalizations and five deaths.