EPIDEMICS, VIRAL

An epidemic is an outbreak of a disease that involves a large number of people in a contained area (e.g., village, city, country). An epidemic that is worldwide in scope is referred to as a pandemic. A number of viruses have been responsible for epidemics. Some of these have been present since antiquity, while others have emerged only recently.

Smallpox is an example of an ancient viral epidemic. Outbreaks of smallpox were described in 1122 B.C. in China. In A.D. 165, Roman Legionnaires returning from military conquests in Asia and Africa spread the virus to Europe. One third of Europe's population died of smallpox during the 15-year epidemic. Smallpox remained a scourge until the late eighteenth century. Then, Edward Jenner devised a vaccine for the smallpox virus, based on the use of infected material from cowpox lesions. Less than a century later, naturally occurring smallpox epidemics had been ended.

Influenza is an example of a viral epidemic that also has its origins in ancient history. In contrast to smallpox, influenza epidemics remain a part of life today, even with the sophisticated medical care and vaccine development programs that can be brought to bear on infections.

Epidemics of influenza occurred in Europe during the Middle Ages. By the fifteenth century, epidemics began with regularity. A devastating epidemic swept through Spain, France, the Netherlands, and the British Isles in 1426–1427. Major outbreaks occurred in 1510, 1557, and 1580. In the eighteenth century there were three to five epidemics in Europe. Three more epidemics occurred in the nineteenth century. Another worldwide epidemic began in Europe in 1918. American soldiers returning home after World War I brought the virus to North America. In the United States alone almost 200,000 people died. The influenza epidemic of 1918 ranks as one of the worst natural disasters in history. In order to put the effects of the epidemic into perspective, the loss of life due to the four years of conflict of World War I was 10 million. The death toll from influenza during 5 months of the 1918 epidemic was 20 million.

Epidemics of influenza continue to occur. Examples include epidemics of the Asian flu (1957), and the Hong Kong flu (1968). Potential epidemics due to the emergence of new forms of the virus in 1976 (the Swine flu) and 1977 (Russian flu) failed to materialize.

The continuing series of influenza epidemics is due to the ability of the various types of the influenza virus to alter the protein composition of their outer surface. Thus, the antibodies that result from an influenza epidemic in one year may be inadequate against the immunologically distinct influenza virus that occurs just a few years later. Advances in vaccine design and the use of agents that lessen the spread of the virus are contributing to a decreased scope of epidemics. Still, the threat of large scale influenza epidemics remains.

In the twentieth century, new viral epidemics have emerged. A number of different viruses have been grouped together under the designation of hemorrhagic fevers. These viruses are extremely contagious and sweep rapidly through the affected population. A hallmark of such infections is the copious internal bleeding that results from the viral destruction of host tissue. Death frequently occurs. The high death rate in fact limits the scope of these epidemics. Essentially the virus runs out of hosts to infect. The origin of hemorrhagic viruses such as the Ebola virus is unclear. A developing consensus is that the virus periodically crosses the species barrier from its natural pool in primates.

Another viral epidemic associated with the latter half of the twentieth century is acquired immunodeficiency syndrome. This debilitating and destructive disease of the immune system is almost certainly caused by several types of a virus referred to as the Human Immunodeficiency Virus (HIV). The first known death due to HIV infection was a man in the Congo in 1959. The virus was detected in the United States only in 1981. Subsequent examination of stored blood sample dating back 40 years earlier revealed the presence of HIV.

HIV may have arisen in Africa, either from a previously unknown virus, or by the mutation of a virus resident in a non-human population (e.g., primates). The tendency of the virus to establish a latent infection in the human host before the appearance of the symptoms of an active infection make it difficult to pinpoint the origin of the virus. Moreover, this aspect of latency, combined with the ready ability of man to travel the globe, contributes to the spread of the epidemic. Indeed, the epidemic may now be more accurately considered to be a pandemic.

A final example of a twentieth century viral epidemic is that caused by the Hanta virus. The virus causes a respiratory malady that can swiftly overwhelm and kill the patient. The virus is normally resident on certain species of mouse. In the mid-1990s, an epidemic of Hanta virus syndrome occurred in native populations in the Arizona and New Mexico areas of the United States west. As with other viral epidemics, the epidemic faded away as quickly as it had emerged. However, exposure of someone to the mouse host or to dried material containing the virus particles can just as quickly fuel another epidemic.

Given their history, it seems unlikely viral epidemics will be eliminated. While certain types of viral agents will be defeated, mainly by the development of effective vaccines and the undertaking of a worldwide vaccination program (e.g., smallpox), other viral diseases will continue to plague mankind.