Emerging infectious diseases have been defined as "infections that have newly existed in a population or have existed but are rapidly increasing in incidence or geographic range". Many are zoonoses, diseases shared with other animals. Globalization of trade, growth of cities and intensification of agriculture are all giving animals new opportunities to spread their pathogens to people. Modern human mobility increases opportunities for the transfer of pathogens, notably between widely separated habitats. The spread of these diseases is notoriously hard to track; mad cow disease, for instance, has such a long incubation period that it may be many years before its existence can be confirmed or denied.
Recent years have seen several episodes of disease emergence, such as multi-drug resistant tuberculosis, acute coccal infections, the rodent-borne pneumonic hantavirus in the United States, food- and water-borne outbreaks of Salmonella infections, cholera and illnesses caused by the Escherichia coli O157. Half a billion people per year develop malaria, and 1 to 2 million die of it, so the disease is re-emerging. The HIV, Ebola and Hanta viruses are examples of diseases that were once apparently confined to Africa and have travelled around the world.
In the first half of the 1990s, a number of communicable diseases re-emerged in the European Region, mainly in the "newley independent states" (NIS) of the former Soviet Union. Although communicable diseases accounted for only 1.3% of all deaths in the region in this period, the mortality rates were four times higher in the NIS than in the rest of Europe. About half the communicable disease mortality in this period was caused by tuberculosis. Insufficient access to safe drinking-water, poor food hygiene, inadequate housing and contamination of indoor air with microorganisms contribute to the disease burden.