Answers may include the following modern demographic and environmental factors that favor the spread of infectious diseases:

Environment (technology and industry )

International travel and commerce, global travel, globalization of the food supply and centralized processing of food; worldwide transport of animals and food products.

Population growth and increased urbanization and crowding; poverty; population movements due to civil wars, famines, and other man-made or natural disasters; migration.

Economic development and land use; irrigation, deforestation and reforestation projects that alter the habitats of disease carrying insects and animals.

Microbial adaptation:development of resistance to newly emerging microbes.

Human behaviors, such as IV drug use and risky sexual behavior; increased use of antimicrobial agents and pesticides, hastening the development of resistance; and increased human contact with tropical forests and other wilderness habitats that are reservoirs for insects and animals that harbor unknown infectious agents; increasing numbers of persons with impaired host defenses.