Despite a substantial fall in fertility rates, especially in developed countries, and a continuing decline in global population growth rates, the momentum of the existing population will increase the world's numbers from 6.1 billion to over 7 billion by 2015. Ninety-five percent of this growth will take place in the Third World, where most of the world's largest cities will contain about half the world's population. In many developing countries, particularly in sub-Saharan Africa, the Middle East, and parts of South Asia, the rapidly increasing number of persons between the ages of fifteen and twenty-four will strain educational systems, infrastructure, and job markets. At the same time, the population of the northern tier states will markedly age. Increasingly, the needs of older persons will impose enormous economic burdens on shrinking workforces. Facing labor shortages, some industrial countries will encourage immigration of both skilled and unskilled labor, as the United States has done. Other countries may prefer to substitute technology for labor or to outsource their labor requirements overseas. Russia's population is likely to decrease substantially, as a result of poor health care and declining birth rates. Russian life expectancy is expected to continue to decline.
Some developing countries will not experience net population growth; despite high birth rates, some African countries that are heavily infected with HIV and other diseases will have stable or even declining populations. Infectious diseases will pose a growing threat fueled by population growth, urbanization, and migration, as well as other factors such as microbial resistance. At the same time that progress is being made with respect to some diseases—such as polio and measles—diseases such as tuberculosis and malaria are re-emerging in deadlier, drug-resistant variations while new infectious diseases appear. It is estimated that at least thirty previously unknown diseases have appeared globally since 1973, including the incurable HIV, hepatitis C, Ebola hemorrhagic fever, and encephalitis-related Nipah. Asia is likely to witness a major increase in infectious disease deaths, replacing Africa as the epicenter of HIV by 2015.