Emerging diseases are commonly defined as illnesses that have increased in incidence during the past two decades or are likely to increase in the near future. Many of these diseases are zoonotic. A zoonotic disease can emerge as the result of increased human contact with the animal host(s), animal tissues, vectors, or environmental sources of the pathogens. It may also result from an increased prevalence of the agent in domesticated or wild animals or in vectors. Many currently emerging and reemerging diseases have reservoirs in wildlife and/or are foodborne.
Factors that can cause emergence include altered human demographics or behavior, ranging from societal upheavals that cause people to leave urban areas to simple changes in food preferences. For example, the popularity of prewashed greens can facilitate some outbreaks of E coli O157:H7. Breakdowns in public health measures such as sanitation and vaccination also increase the spread of disease. Changing land use patterns may alter the number of reservoir hosts, increase the incidence of infection in these animals, encourage genetic changes in the pathogen (eg, recombination with other strains), or bring animal hosts or disease vectors into closer contact with people. Because many mosquitoes preferentially breed along the edges of forests rather than deep among the trees, deforestation can increase populations of these species. Degradation of their natural habitats, as well as the ready availability of food near human dwellings, can encourage wildlife to move into suburban areas. The growth of the human population also exerts pressures that ultimately result in increased contact with wildlife. Climate change can be a factor in disease emergence, particularly for arthropodborne pathogens such as Rickettsia spp. A warmer climate not only allows vectors to survive the winter but also permits a longer transmission season. Climate changes may also favor some non-arthropodborne diseases; for example, increased rainfall has been linked to outbreaks of plague in some areas.
Technologic and industrial changes in food production can contribute to disease emergence by increasing the concentration, movement, and mixing of animals. Long-distance transport has been associated with increased shedding of enteric pathogens, including Salmonella. Decreased genetic diversity may eliminate species, breeds, or individuals with innate resistance to a disease. The development of large-scale farms and food-processing facilities has led to the exposure of greater numbers of people to a contaminated food source. Increased mobility of people, animals, and goods allows diseases to spread quickly. Viruses that formerly died out after affecting small numbers of animals and/or people can now find many susceptible hosts within a short period. The SARS coronavirus spread to 30 countries on 6 continents within months of the initial outbreak. In late 2019 a novel coronavirus, SARS-CoV-2, was first detected in China and within 4 months had infected millions of people around the world. Read more about the disease it causes, COVID-19, in the MSD Manual, Professional Edition. Occasionally, a pathogen may become more virulent or better adapted to people, or it may undergo changes that affect transmission patterns. More virulent strains of the West Nile virus have emerged since the 1980s, and a new variant, which can replicate more rapidly in mosquitoes, has become established in North America.
Some diseases are emerging not because they are more common but because they are better recognized. Increased recognition can result from improved diagnostic techniques, increased use of laboratories for identification of specific pathogens, and better awareness among physicians. Some spotted fever Rickettsia spp are emerging, in part, because the increased use of molecular techniques facilitates their identification. Marburg virus, once thought to be a very rare and less virulent relative of ebolaviruses, was recently found to have caused hemorrhagic disease in workers at one African mine since the 1980s or earlier. This focus of bat-transmitted infections was only recognized when highly fatal outbreaks affected hundreds of people in the Democratic Republic of the Congo in 1998–2000.
Increased human susceptibility has contributed to the emergence or recognition of some opportunistic pathogens. The number of immunocompromised people has been increased by factors such as the AIDS epidemic, the success of organ transplantation programs, and the improved survival of those with primary and secondary immunodeficiencies. Modern medicine also allows more people, many of whom develop chronic conditions, to survive to an advanced age.