The combination of population growth (especially in cities), international air travel, incessant migration and the ebb and flow of refugees means that the peoples of the world are more intermingled now than at any time in history. Thus human transmission could become the predominant way in which diseases are spread quickly, not just from person to person but from continent to continent - by airborne and droplet spread, sexual transmission, bloodborne transmission or direct contact.
In children, the major diseases disseminated by airborne and droplet spread are acute respiratory infections, particularly pneumonia, influenza, measles, pertussis (whooping cough), meningococcal meningitis and diphtheria, which together kill at least four million. Direct contact diseases in children include poliomyelitis and trachoma, a major cause of blindness in developing countries. Among adults, tuberculosis is the leading airborne disease, killing three million people and infecting almost nine million others every year. It is already the opportunistic infection that most frequently kills HIV-positive people: of an estimated one million AIDS-related deaths in 1995, about one-third may have been due to tuberculosis. Leprosy still affects 1.8 million people in 70 countries, but is steadily being eliminated as a public health problem. Influenza and pneumonia strike children and adults, especially the elderly.
Of all sexually transmitted diseases, HIV/AIDS continues to have the greatest global impact, with an estimated 20 million adults currently affected. In addition to HIV, at least 333 million new cases of other sexually transmitted diseases occurred in 1995.
Among bloodborne infections, hepatitis causes most concern. More than 2000 million people alive today have been infected with hepatitis B; some 350 million are chronically infected and thus at risk of serious illness and death from liver cirrhosis and liver cancer. In addition, some 100 million are chronically infected with the hepatitis C virus. Unlike hepatitis B, there is no vaccine for hepatitis C.