Why are new infectious diseases emerging?
The reasons for the sharp increase in incidence of manyinfectious diseases – once thought to be under control – arecomplex and not fully understood. Population shifts andpopulation growth; changes in human behavior; urbanization,poverty, and crowding; changes in ecology and climate; theevolution of microbes; inadequacy of public healthinfrastructures; and modern travel and trade have allcontributed. For example, the ease of modern travel creates manyopportunities for a disease outbreak in remote areas to spread toa crowded urban area. Human behavioral factors, such as dietaryhabits and food handling, personal hygiene, risky sexualbehavior, and intravenous drug use can contribute to diseaseemergence. In several parts of the world, human encroachment ontropical forests has brought populations with little or nodisease resistance into close proximity with insects that carrymalaria and yellow fever and other, sometimes unknown, infectiousdiseases. In addition, local fluctuations in temperat
