Epidemics, viral WORLD OF MICROBIOLOGY AND IMMUNOLOGY196•
the series of inoculations to establish immunityto the infection
readily kill all three types of bacteria. But, in regions where
such preventative measures are not practiced, meningitis epi-
demics are a problem. In 1996, the largest meningitis epidemic
ever recorded, in terms of the numbers of affected people,
occurred in West Africa. An estimated 250,000 people con-
tracted meningitis and 25,000 people died of the infection.
Leprosyis an example of a bacterial epidemic that used
to be common and which is now on the way to being elimi-
nated. The disease is caused by Mycobacterium leprae. The
bacterium was discovered by G.A. Hansen in 1873, and was
the first bacterium to be identified as a cause of human disease.
Epidemics of leprosy were common in ancient times;
indeed, During the first century A.D., millions of people were
afflicted with the disease. Nowadays, the number of leprosy
patients in the entire world has been reduced some ninety per-
cent over earlier times through a concerted campaign of diag-
nosis and treatment that began in the 1990s. Still, leprosy
remains at epidemic proportions in six countries: Brazil, India,
Madagascar, Mozambique, Myanmar, and Nepal. In these
countries an estimated half million new cases of leprosy
appear every year.
In contrast to leprosy, tuberculosisis an epidemic that is
increasing in prevalence with time. Tuberculosis is caused by
another mycobacterium called Mycobacterium tuberculosis.
The lung infections caused by epidemics of tuberculosis kill
two million people each year around the world. The number of
cases of tuberculosis is growing because of the difficulty in
supplying health care to some underdeveloped areas, the
increase of immunocompromising diseases such as Human
ImmunodeficiencySyndrome, and the appearance and spread
of a strain of the bacterium that is resistant to many of the drugs
used to treat the infection. Estimates from the World Health
Organizationindicate that if the tuberculosis epidemic contin-
ues nearly one billion people will become infected by 2020. Of
those, some 35 million people will die of tuberculosis.
The re-emergence of tuberculosis is paradoxical.
Whereas other bacterial epidemics have and are being con-
trolled by modern methods of treatment, such methods are
exacerbating the tuberculosis epidemic. Part of this is also due
to the target of the bacterial infection. Lung infections are
harder to conquer than infections of the skin, such as occurs in
leprosy. Moreover, when the immune systemis not function-
ing properly, due to the presence of another infection, the lung
infection can become deadly. Thus, tuberculosis is an example
of a bacterial epidemic whose scope is changing with the
emergence of other infections and treatments.
The tuberculosis epidemic also underscores the danger
of ineffective treatment and the effect of modern life on the
spread of disease. Poorly supervised and incomplete treatment
has caused the emergence of the drug-resistant strains of the
bacteria. The bacteria can remain in the lungs and so can infect
others. With the greater movement of people around the globe,
the spread of the disease by carriers increases.
A final example of a bacterial epidemic is cholera. The
disease caused by Vibrio choleraeis an example of an ancient
bacterial epidemic that continues today. The intestinal infec-
tion produces a watery diarrhea that can lead to a fatal dehy-dration. An epidemic of cholera caused by an antigenic ver-
sion of Vibrio choleraeknown as El Tor has been in progress
since 1961. Indeed, the various epidemics are so widespread
geographically that the disease can be considered pandemic (a
simultaneously outbreak of illness on a worldwide scale). The
latest epidemics have included countries in West Africa and
Latin America that had been free of cholera for over a century.
Cholera is spread by contaminated food or water. Thus,
the sanitary condition of a region is important to the presence
of the epidemic. As with other bacterial epidemics of the past
and present, underdeveloped regions are the focus of epidemic
outbreaks of cholera.See alsoBacteria and bacterial infection; Biological warfare;
Vaccination; Water qualityEEpidemics, viralPIDEMICS, VIRAL
An epidemic is an outbreak of a disease that involves a large
number of people in a contained area (e.g., village, city, coun-
try). An epidemic that is worldwide in scope is referred to as a
pandemic. A number of viruseshave been responsible for epi-
demics. Some of these have been present since antiquity,
while others have emerged only recently.
Smallpoxis 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 con-
quests 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 eigh-
teenth century. Then, Edward Jennerdevised a vaccinefor the
smallpox virus, based on the use of infected material from
cowpoxlesions. Less than a century later, naturally occurring
smallpox epidemics had been ended.
Influenzais 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 sophisti-
cated 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 cen-
tury. 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 epi-
demic was 20 million.
Epidemics of influenza continue to occur. Examples
include epidemics of the Asian flu (1957), and the Hong Kongwomi_E 5/6/03 2:12 PM Page 196