Diseases Essay, Research Paper
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 frequently.
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 the
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
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 sexual contact.
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 water.
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 habits.
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
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 digest it.
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
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 confront us.