Diseases are any harmful change that interferes with the
normal appearance, structure, or function of the body or
any of its parts. Since time immemorial, disease has played
a role in the history of societies. It has affected and has
been affected by economic conditions, wars, and natural
disasters. An epidemic of influenza that swept the globe in
1918 killed between 20 million and 40 million people.
Within a few months, more than 500,000 Americans died^more
than were killed during World War I (1914-1918), World War
II (1939-1945), the Korean War (1950-1953), and the Vietnam
War (1959-1975) combined.
Diseases have diverse causes, which can be classified into
two broad groups: communicable and noncommunicable.
Communicable diseases can spread from one person to another
and are caused by microscopic organisms that invade the
body. Noncommunicable diseases are not communicated from
person to person and do not have, or are not known to
involve, infectious agents. Some diseases, such as the
common cold, and come on suddenly and last for no more than
a few weeks. Other diseases, such as arthritis, are
chronic, consistent for months or years, or reoccur
Every disease has certain characteristic effects on the
body. Some of these effects, include fever, inflammation,
pain, fatigue, dizziness, nausea, and rashes, are evident
to the patient. These symptoms offer important clues that
help doctors and other health care professionals make a
diagnosis. Many times, the symptoms point to several
possible disorders. In those cases, doctors rely on medical
tests, such as blood examinations and X rays, to confirm
Communicable diseases are caused by microscopic organisms.
Physicians refer to these disease-causing organisms as
pathogens. Pathogens that infect humans include a wide
variety of bacteria, viruses, fungi, protozoans, and
parasitic worms. Also, it has been theorized that some
proteins called prions may cause infectious diseases.
Bacteria are microscopic single-celled organisms at least 1
micron long. Some bacteria species are harmless to humans,
many are beneficial. But some are pathogens, including
those that cause cholera, diphtheria, leprosy, plague,
pneumonia, strep throat, tetanus, tuberculosis, and typhoid
fever. The bacteria that are harmless and live in or on you
are called resident bateria.
Viruses are tens or hundreds of times smaller than
bacteria. They are not cellular, but consist of a core of
genetic material surrounded by a protective coat of
protein. Viruses are able to survive and reproduce only in
the living cells of a host. Once a virus invades a living
cell, it directs the cell to make new virus particles.
These new viruses are released into the surrounding
tissues, and seek out new cells to infect. The roll call of
human diseases caused by viruses includes mumps, measles,
influenza, rabies, hepatitis, poliomyelitis, smallpox,
AIDS, and certain types of cancer.
Fungi are a varied group of generally small organisms that
get their food from living or dead organic matter. They
germinate from reproductive cells called spores, which
often have a thick, resistant outer coat that protects
against unfavorable environmental conditions. This enables
spores to survive for long periods of time, which adds to
the difficulty of treating fungal infections. Some fungi
are external parasites of humans, causing skin conditions
such as ringworm, athlete's foot, and jock itch. Other
fungi invade internal tissues. Examples include yeast that
infect the genital tract and several fungi species that
cause a type of pneumonia.
Protozoans are single-celled, animal-like organisms that
live in moist environments. The most infamous pathogenic
protozoans are species of the genus Plasmodium, which cause
malaria, an infectious disease responsible for over 2
million deaths worldwide each year. Members of the genus
Trypanosoma produce trypanosomiasis, also known as African
sleeping sickness, and Chagas' disease. Other protozoans
cause giardiasis, leishmaniasis, and toxoplasmosis.
Some pathogens are spread from one person to another by
direct contact. They leave the first person through body
openings, mucous membranes, and skin wounds, and they enter
the second person through similar channels. The viruses
that cause respiratory diseases such as influenza and the
common cold are spread in moisture droplets when an
infected person coughs or sneezes. A hand that was used to
cover the mouth while coughing contains viruses that may be
passed to doorknobs, so that the next person to touch the
doorknob has a chance of picking up the infectious agent.
The bacteria that cause some sexually transmitted diseases,
including gonorrhea and syphilis, are transmitted during
Other pathogens involve an intermediary carrier, such as an
insect. The malarial parasite, for example, spends part of
its life cycle in mosquitoes, then enters a person's
bloodstream when the mosquito bites the person. Many
pathogens are spread through contaminated food and water.
Other pathogens can be passed on by contaminated food or
Noncommunicable diseases not known to be caused by
infectious agents include the three leading killers in the
United States and other developed countries: heart disease,
most cancers, and cerebrovascular disease. Noncommunicable
illnesses include disorders as terrifying as Alzheimer's
disease, which robs victims of their memory and their
ability to reason, and as pesky as poison ivy.
Degenerative disorders, including arthritis, Parkinson's
disease, and Alzheimer's disease, involve the progressive
breakdown of tissues and loss of function of parts of the
body. Joints gradually become stiff; bones become brittle;
blood vessels become blocked by deposits of fat. The
incidence of these problems increases with age, and, in at
least some cases, progression can be slowed by good health
There are many ways to prevent these diseases. The skin and
mucous membranes form the body's first line of defense
against disease. Most microscopic pathogens, or microbes,
cannot pass through unbroken skin, although they can easily
enter through cuts and other wounds. Mucous membranes
protect internal organs that are connected with the outside
of the body. These membranes, which line the respiratory,
digestive, urinary, and reproductive tracts, secrete mucus,
which traps microbes. The mucus may then be expelled from
the body, perhaps in a cough or sneeze or in feces. If the
mucus is swallowed, digestive juices kill the microbes.
Small hairlike projections on the lining of the nose,
throat, and bronchial tubes work in conjunction with mucus
to trap and remove foreign substances. In the ears, tiny
hairs plus a sticky wax defend against the entry of germs.
Tears secreted by the lachrymal gland wash away germs and
other small objects that may enter the lid area of the eye.
Tears also contain a protein that kills certain germs.
If a pathogen breaks through the body^s outer barriers, the
defenses of the immune system spring into action. Some of
these defenses are effective against a variety of invaders,
while others are formed to fight a specific organism. White
blood cells called phagocytes constantly travel through the
bloodstream on the lookout for foreign objects. If they
come upon a microorganism, they surround, engulf, and
During the 20th century, the importance of vitamins and
other nutrients in preventing disease was recognized.
Antibiotics, sulfa drugs, blood types, and genes that cause
disease were discovered. A host of diagnostic and surgical
tools were created that incorporated inventions such as X
rays, fiber optics, lasers, and computers. Techniques such
as organ transplantation, kidney dialysis, dental implants,
gene therapy, and fetal surgeries were introduced.
Thousands of new drugs were developed to treat everything
from ulcers to zinc malabsorption.
At the beginning of the 20th century, people in the United
States had an average life span of about 50 years. By the
time the century neared its close, average life span had
risen to 76 years. Other developed countries experienced
similar increases. Much of the credit for these longer life
spans, and for the good health that accompanies them, is
due to the conquering of diseases, thanks to vaccines,
antibiotics, sophisticated surgical tools, and other
medical miracles. The challenges ahead include bringing the
benefits of this medical knowledge to all peoples of the
world, and expanding on current knowledge in order to
understand, treat, and prevent the diseases that still