Dengue is an infectious disease, which means that it is caused by tiny
pathogenic (harmful) organisms and can be contagious, or spread among
people upon contact with an infected person.
Humankind has always been afflicted by infectious diseases, but epidemics
(sudden outbreaks of disease in great numbers) were comparatively
rare before the advent of civilization. The primary reasons for the
lack of infectious disease before this time are the following:
• Individual hunters and gatherers led a nomadic existence rather than
living in large groups with other people, and this limited the spread
of disease.
• The absence of domesticated animals such as horses and cows limited
the spread of disease.
Eventually, most people stopped leading a nomadic life and began
living near one another. Perhaps one of the reasons for this was the extinction
of larger, wild mammals due to hunting, which meant that people
needed to rely on domesticated animals and cultivated plants for food. As
more people lived together and began raising animals and crops for food,
infectious diseases emerged as a result of this close proximity. During this
time the following occurred:
• Primary food sources changed from wild plants and animals, birds,
and fish to cultivated plants and domesticated animals.
• Large numbers of families lived close to one another.
An Introduction to
Infectious Disease
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An Introduction to Infectious Disease 9
• The development of agriculture resulted in poorer, carbohydrate-
rich diets with consequent under-nutrition and
less individual resistance to infections.
• Infectious diseases became one of the principal reasons
for limitations on the size of populations.
Diseases spread easily as human populations became concentrated
in villages and then in towns and cities. People and
animals also often lived under the same roof, so people were
exposed to influenza, salmonella, and tuberculosis infections,
and parasitic worms. People were exposed to diseases either by
direct contact (through the air, polluted water, and food) or
by indirect contact (through blood-sucking insect carriers of
disease such as mosquitoes, fleas, and lice). Moreover, as traders,
invading marauders, and armies traveled from city to city,
they introduced diseases and the animal and insect carriers of
diseases. Because a balance between infective organisms and a
local population was often eventually achieved, human carriers
of infection, who often were not ill themselves, became important
sources of spreading disease when they came into contact
with nonimmune populations.