The extent to which contagious and infectious diseases spread through human populations is highly variable. In a developed country a few hundred cases of typhoid or plague may produce anxiety or even mild panic, but the outbreak can usually be controlled by routine public health measures. Such an epidemic is not a threat to the survival of the community. In developing countries, diseases may be much more widespread and have a regular endemic character; in these cases, massive spread to the whole community is limited by a high degree of naturally acquired immunity, and public health measures may help. On the other hand, diseases (particularly new strains) like influenza or measles may attack a large proportion of a community, especially if it has acquired little immunity from previous exposure or by immunization. That some diseases (for example, influenza, measles and plague) are in fact capable of affecting a large part of a population in a highly disastrous fashion is a matter of historical fact.
A potential pandemic virus would have three hallmarks. It would be of a new type to which large numbers of people would have no immunity. The illness it causes would be serious and life threatening, and it would be capable of spreading quickly from person to person.
A major question is whether epidemics are rare accidents or whether they could be started deliberately. It is possible that, by a deliberate selection of mutants or through recombinants, highly virulent and spreading strains could be obtained. By 1998, research had shown that many new highly virulent forms of influenza arise due to antigenic shift. The antigens of the influenza change, either through mutation or through assembly of a new combination of animal viruses. Ducks can store both bird viruses and some forms of swine flu virus, and some of these in combination can infect humans, such as the modern H5N1 (Hong Kong 1968) virus. Thus epidemics can be due to accidents, but it is highly likely that such accidents will continue to occur frequently.
The influenza virus epidemic of 1918, the final year of World War I, was the worst pandemic in history. Only bubonic plague ("Black Death") can compare. The Black Death appeared three times – in the 6th, 14th and 17th centuries – and killed an estimated at 137 million. But the 1918 flu, misnamed "Spanish", outstripped the plague with its speed and high death rate among its victims. At its worst, the Black Death claimed 2 million lives a year. The 1918 flu caused nearly 20 times that many deaths in less than a year and a total of close to 40 million worldwide.
In the 1960s, communicable diseases that can spread epidemically were played down by health authorities because it was assumed that new antibiotics, better vaccines and improved sanitation would eradicate them. In fact, new microbes have emerged, and well-known illnesses are not under control; they are re-emerging.
The rise of AIDS in Kalingrad, Russia in 1997 has been epidemic. In 1996 28 people were known to have been infected, including 5% of the prostitutes. One year later there were over 1,850 cases of HIV, and 85% of the prostitutes had been infected. Drug abuse is common, and people deny AIDS is a problem, as they have not seen sick AIDS victims.