FREE ESSAY ON HUMAN DISEASE RESEARCH

HUMAN DISEASE RESEARCH

Human Disease
IINTRODUCTION Human Disease, in medicine, 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
been affected by-economic conditions, wars, and natural disasters. Indeed, the impact of
disease can be far greater than better-known calamities. 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: infectious
and noninfectious. Infectious diseases can spread from one person to another and are
caused by microscopic organisms that invade the body. Noninfectious 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, are acute, coming on suddenly
and lasting for no more than a few weeks. Other diseases, such as arthritis, are chronic,
persisting for months or years, or recurring frequently.
Every disease has certain characteristic effects on the body. Some of these effects,
called symptoms and signs, include fever, inflammation, pain, fatigue, dizziness, nausea,
and rashes, and are readily apparent to the patient. These symptoms offer important clues
that help physicians and other health care professionals make a diagnosis. Many times,
however, 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 diagnosis.
The course of a disease-that is, the path it follows from onset to end-can vary
tremendously, depending largely on the individual and the treatment he or she receives.
For example, otherwise healthy people usually recover quickly from a bout of pneumonia if
given proper treatment, whereas pneumonia often proves fatal to people with a weakened
immune system and to those who do not receive prompt, effective treatment. Some diseases
run a different course depending on the patient's age. Chicken pox, for instance, is
usually mild in childhood but severe in adults. In the United States, only about 5
percent of chicken pox cases occur in people over the age of 20, but these cases account
for 50 percent of all deaths from the disease.
Scientists, public health officials, and other members of the medical community work
diligently to try to prevent disease epidemics. The battle is constant and is fought on
many fronts. There have been many victories. Once-devastating diseases such as smallpox
and diphtheria have been virtually eradicated, and many other diseases that once
conferred automatic death sentences can now be either cured or controlled. At the same
time, however, new killers have emerged. Acquired immune deficiency syndrome (AIDS) and
hantavirus pulmonary syndrome are among at least 30 diseases that have been identified by
scientists since the early 1970s. Other growing challenges, particularly in the affluent
societies of industrialized nations, are so-called diseases of choice, such as alcohol
abuse, drug abuse, or obesity, that result from addictive behavior, poor eating habits,
or insufficient exercise.
Complicating matters further are societal changes. Increased international travel
accelerates the spread of both new and old diseases: A person infected with an unusual
virus on one continent can arrive-with the virus-on another continent in a matter of
hours. Ships, planes, and trucks can transport disease-carrying organisms just as easily.
In 1985 tires imported into Texas from Asia carried larvae of the Asian tiger mosquito,
which is a carrier of dengue fever and other tropical diseases. Within five years, Asian
tiger mosquitoes were living in 17 states. Changing dietary habits and the availability
in local supermarkets of foods from all parts of the world contribute to an increase in
food-borne illnesses. Some researchers worry that growing populations and the resulting
crowded living conditions will increase the risk of epidemics.
IIINFECTIOUS DISEASE Infectious diseases are caused by microscopic organisms commonly
called germs. 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. In addition, it has been theorized that some proteins called prions may
cause infectious diseases.
APathogens Bacteria are microscopic single-celled organisms at least 1 micron long. Most
bacteria species are harmless to humans; indeed, many are beneficial (see eubacteria).
But some are pathogens, including those that cause cholera, diphtheria, leprosy, plague,
pneumonia, strep throat, tetanus, tuberculosis, and typhoid fever.
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.
Perhaps 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.
Parasitic flatworms include tapeworms, which live in the intestines of a host organism.
They have a ribbon-like body that may be up to 9 m (30 ft) in length, depending on the
species. Hooks and suckers on the head attach a tapeworm to the intestinal wall, and a
tough outer coating protects against the host's digestive juices. Another group of
parasitic flatworms is flukes, which are responsible for several serious tropical
diseases, most notably schistosomiasis. See Parasite.
Roundworms, or nematodes, are small, tubelike worms that are pointed at both ends.
Species that infect human intestines include pinworms, hookworms, threadworms, and
members of the genus Ascaris. Trichinella spiralis can invade human muscle tissue, often
from eating infected pork that has been improperly prepared, causing a disease called
trichinosis.
Prions are extremely tiny protein particles found in the brain, nerve, and muscle cells.
A controversial theory states that prions cause disease by changing normal proteins into
an abnormal shape. These mutated proteins in turn force other proteins to change shape,
leading to destruction of tissue, primarily in the brain. Some researchers have
hypothesized that prions cause transmissible spongiform encephalopathies, a group of rare
infectious diseases that includes Creutzfeldt-Jakob disease in humans, scrapie in sheep,
and bovine spongiform encephalopathy (commonly known as mad cow disease) in cattle. Some
evidence suggests that prion-related disease can be transmitted through food infected
with mutated proteins.
BSpread of Infectious Disease 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. For example, 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. Cholera bacteria, for example, are spread through
food and water contaminated with the excrement of infected people.
CNew Infectious Diseases In 1978 the United Nations adopted a resolution that set goals
for eradicating infectious disease by the year 2000. This lofty goal proved impossible to
achieve. The years since the resolution was adopted have seen the emergence of new
killers and a rise in the incidence of such ancient scourges as malaria, yellow fever,
and tuberculosis.
Among the diseases new to science are AIDS, Ebola hemorrhagic fever, Legionnaires'
disease, and Lyme disease. AIDS has been the most deadly of all the new diseases, but
even it has not taken as high a toll as malaria, tuberculosis, and other diseases that
have been around for centuries. Some newly identified disease-causing agents for diseases
that have been recognized for a long time include Human T-lymphotropic virus I (HTLV-1),
which can cause some cases of non-Hodgkin's lymphoma, a type of cancer originating in the
lymphatic system; and HTLV-2, which is associated with hairy-cell leukemia, a rare type
of cancer of the blood.
In most cases, the reasons for the emergence of a new disease are unknown. One exception
is Legionnaires' disease. It is caused by a bacterium that was not identified until after
an outbreak in 1976 at an American Legion convention in Philadelphia, Pennsylvania. Once
identified, however, scientists were able to retrospectively identify earlier epidemics
of the disease, and realized that each year the bacterium is responsible for thousands of
cases of pneumonia.
Environmental changes may be responsible for some new diseases. Scientists speculate that
the viruses for some of the deadly hemorrhagic fevers that have surfaced in Africa, such
as Ebola and Marburg disease, have long existed in certain wild animals. As people have
encroached on wilderness areas they have come into contact with the infected animals, and
the viruses have jumped from their traditional animal host to a new human host, with
deadly consequences.
In addition to new diseases, well-known pathogens may change, or mutate, creating new,
virulent strains. Influenza viruses are among those that mutate frequently, which
explains why flu shots-vaccines that use modified or killed versions of the influenza
agent to stimulate a protective immune response in the body-are given annually, and why
epidemics of influenza periodically occur. The strains of flu virus that were most
prevalent one year differ from those that bedevil humans the next year. Vaccines that
protected against last year's flu virus may need to be altered to be effective against
today's most common strains.
A similar problem occurs when mutations in infectious agents result in resistance to
medicines that had been effective treatments. The bacteria that cause bronchitis,
meningitis, tuberculosis, and pneumonia are among many that have developed strains that
are resistant to at least some antibiotics. As a result, doctors have fewer options for
treating the diseases and preventing their spread.
IIINONINFECTIOUS DISEASE 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 (decreased blood circulation in the brain).
Noninfectious 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 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 (see Aging), and, in
at least some cases, progression can be slowed by good health habits.
Environmental factors play critical roles in numerous noninfectious diseases. Exposure to
carbon monoxide can have long-term effects on the heart and vision. Lead in drinking
water can impair children's mental abilities and increase blood pressure in adults.
Occupational exposure to coal dust, cotton dust, and asbestos predisposes workers to
black lung, brown lung, asbestosis, and other respiratory diseases (see Occupational and
Environmental Diseases). Other diseases are caused by an addiction to a harmful
substance. Tobacco smoking is a prime culprit in emphysema, as well as lung cancer and
other respiratory diseases. Excessive use of alcohol can lead to liver disease, brain
damage, and nutritional disorders.
Repetitive stress injuries result from repeating certain motions, usually from a fixed or
awkward posture. Twisting items on a factory assembly line, carrying bulging mailbags,
using vibrating tools such as pneumatic hammers, or practicing the piano or a tennis
stroke for hours on end can all result in pain, inflammation, and permanent nerve
damage.
AHereditary and Congenital Diseases Hereditary diseases such as hemophilia, sickle-cell
anemia, Huntington's disease, muscular dystrophy, and Tay-Sachs disease are caused by
mutated genes inherited from one or both parents (see Genetic Disorders). Certain other
diseases, such as diabetes mellitus, hypertension, and some types of cancer, often run in
families, which suggests that heredity is at least partially responsible for their
development.
Congenital diseases, or birth defects, are disorders that are present at birth. Some are
hereditary, others develop while a baby is in its mother's uterus or during the process
of delivery. For example, if the mother contracts German measles, or rubella, during the
early stages of pregnancy, her child may be born with heart defects, eye cataracts,
deafness, or mental retardation. Use of alcohol during pregnancy can cause fetal alcohol
syndrome, characterized by mental and physical retardation. Abnormal development of any
body part in a fetus may produce a congenital defect; for example, if walls that separate
the chambers of the heart fail to form completely, the baby is born with congenital heart
disease.
BImmunological Diseases Immunological diseases occur when the immune system, which
normally protects against infections, malfunctions. The most common types of
immunological diseases are allergies, autoimmune diseases, and immune deficiencies.
An allergy is an abnormal reaction of the immune system to foreign substances, such as
plant pollen, fungal spores, animal danders, medications, and foods. Rhus dermatitis is
an allergy caused by contact with urushiol, an oil resin produced by poison ivy, poison
oak, and poison sumac.
Autoimmune diseases develop when the immune system goes awry and attacks the body's own
tissues. Autoimmune disorders includes lupus erythematosus, rheumatoid arthritis,
juvenile-onset diabetes, and myasthenia gravis. The causes are unknown, although some
scientists suspect the diseases may be triggered in some cases by a pathogen, such as a
virus, or other environmental factor.
Immune deficiency diseases develop when the immune system becomes impaired, resulting in
more common, frequent, or severe infections. The immune system may be damaged by a
genetic abnormality or by illness, injury, the use of a strong drug such as those used in
chemotherapy, or malnutrition.
CDeficiency Diseases Deficiency diseases result from insufficient amounts of various
healthful nutrients in the diet. Examples include scurvy, caused by a deficiency of
vitamin C, or ascorbic acid; pellagra, caused by a deficiency of niacin; and
osteoporosis, caused at least in part by a lack of calcium. Deficiency diseases are most
prevalent in poverty- or war-stricken areas of the world, where malnutrition is
widespread. Deficiency diseases are also found in more affluent nations where food is
prevalent but people's food choices or behavior do not provide well-rounded nutrition,
resulting in such disorders as anorexia nervosa, bulimia, and anemia.
IVTHE FIGHT AGAINST DISEASE ANatural Defenses 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 a sticky fluid called 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 breaches 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 tailor-made 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. 
If the infection persists and there are too many organisms for the phagocytes to fight by
themselves, the immune system produces proteins called antibodies. Each antibody is
designed to combat a particular antigen, or foreign protein. Two types of white blood
cells are involved in this process. B cells release the antibody, which attaches to the
outer covering of the antigen, marking it for destruction. T cells attack the tagged
antigen and also stimulate B cells into action. Once the body has produced antibodies to
a specific microorganism, it generally is immune to future invasions by that organism.
That is why people who have had chicken pox or measles as a child will not get the
disease again as an adult. The reason people get one cold after another is that each cold
is caused by a different virus strain.
BMedical Defenses Much of early medicine was practiced by trial and error, but ancient
peoples also looked for causes and cures for disease by studying the body and observing
the sick. In Greece during the 5th century BC, the physician Hippocrates stressed that
medical care was a science that could be learned through clinical observation and
experimentation. The connection between health and hygiene was made in several ancient
cultures, including those of India and Rome. The Romans drained marshes where
malaria-carrying mosquitoes bred, and they built underground sewers and aqueducts to
carry clean water in the cities. Laws governed the cleanliness of streets and the storage
of food.
Because of limited contact between cultures, most early knowledge of the efficacy of
various measures did not spread from place to place. With the collapse of the Roman
Empire around AD 400, much medical knowledge was lost, to be replaced by superstition. It
was not until the 14th century that a medical renaissance began. Thereafter, progress
occurred exponentially. Accurate descriptions of the structure and functioning of the
human body were made, and the invention of the printing press in the middle of the 15th
century enabled this information to be published and easily disseminated. The development
of microscopes in the late 16th century prompted the discovery of microorganisms,
although it was not until the 19th century that scientists were able to show that
bacteria and other microbes caused disease. Also in the 19th century, people recognized
the importance of sanitation and cleanliness, improving the survival rate in hospitals.
Anesthesia was discovered and the first vaccines were produced.
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 (see Medical 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.
The list of medical techniques for fighting disease continues to grow. More effective
methods are expected to be introduced in the coming years as scientists gain a better
understanding of such subjects as the molecular biology of normal and abnormal cells,
gene structure and action, and the relationship between environmental stresses and
disease.
VPREVENTING DISEASE It is much less costly, in terms of both human suffering and
economics, to prevent disease than to treat it. Public health services and medical
professionals play critical roles in helping people avoid disease. In addition, each
individual plays a vital role in protecting his or her personal health.
Public health services are charged with protecting community health. Their activities
include provision of adequate clean water and the sanitary disposal of sewage and other
wastes. Food supplies-on farms, at food processing plants, and in supermarkets and
restaurants-are inspected for microorganisms. Pesticide spraying programs are undertaken
to control populations of mosquitoes and other carriers of disease. Public facilities,
such as schools and hospitals, are inspected to ensure that they meet appropriate
standards of cleanliness and safety. Education and surveillance programs alert physicians
and other medical workers to disease threats.
Physicians, dentists, and other medical experts have a number of preventive tools at
their disposal. Among the most effective are vaccines, which stimulate the immune system
to produce antibodies against particular antigens. A vaccine may contain killed or
weakened pathogens, parts of the pathogens, or modified toxins produced by the pathogens,
which are strong enough to arouse the immune system to fight off new invading pathogens
but not powerful enough to cause disease themselves. Thanks to vaccines, polio is rare
today, smallpox has been eliminated, and diseases such as diphtheria and whooping cough,
which once killed many young children, have largely been brought under control.
Regular medical check-ups are another important preventive tool. These help doctors to
find disease in its early stages, when it is easier to treat and before it causes
significant damage. For example, during a check-up a dentist will remove plaque, a sticky
bacterial coating on teeth. Left undisturbed in hard-to-reach areas, such as between the
teeth and along the gums, plaque can lead to periodontal disease, which can destroy the
tissues that anchor the teeth in the mouth (see Dentistry).
Even the finest public health and medical services are of limited value to people who
have poor health habits. Numerous studies have proven that physical health and longevity
are linked to the following: eating a balanced diet, maintaining proper weight,
exercising regularly, using condoms and limiting the number of sexual partners, avoiding
tobacco, and avoiding alcohol or consuming it in moderation. People who fail to follow
these guidelines increase their risk of cardiovascular disease, cancer, AIDS, hepatitis,
and other lethal diseases.
The interplay among public health measures, medical practices, and personal
responsibility is exemplified in the fight against tooth decay. Caused by bacteria that
feed on food debris in the mouth, tooth decay can be virtually eliminated through a
combination of three steps: the addition of fluoride to public drinking water supplies;
the professional coating of teeth with a plastic sealant, which fills microscopic pits
where bacteria can collect and cause decay; and regular brushing and flossing of teeth.
VIHISTORY OF HUMAN DISEASE Humans have always had to deal with disease. Skeletons more
than 12,000 years old show evidence of tuberculosis and other diseases. The 9400-year-old
mummified remains of Spirit Cave man, found in Nevada in 1940, indicate that he suffered
from back problems and tooth abscesses. The remains of Ramses V, ruler of Egypt around
1150 BC, show that his face was disfigured by smallpox scars.
Disease has had a dramatic impact on human history. For most of the 250,000 years that
humans have been on the earth, disease has played a central role in limiting population
growth. As ways to combat disease were discovered, people lived longer and had more
children, who lived long enough to have children of their own. The human population
slowly increased and then exploded. By 1804 the human population reached 1 billion. Just
over 100 years later, in 1927, after the advent of the first vaccines and the recognition
of the importance of sanitation and safe water supplies, the population had doubled to 2
billion. By 1974 it had doubled again to 4 billion. Since then, recognition that the
earth's environment has a limited capacity to support an ever-increasing population has
led to concerted efforts to limit population growth. Nevertheless, as the 20th century
neared its end, the population had reached 6 billion. It is expected to rise to more than
8 billion by 2021.
AEpidemics Periodically, devastating outbreaks of infectious disease occur, affecting
many people in a region at the same time. Such outbreaks are called epidemics. Those of
widespread proportions, such as the current AIDS epidemic, are often referred to as
pandemics.
People have always been fearful of epidemics and their effects. In China in the 13th
century BC, the ruler of Anyang asked his diviners, Will this year have pestilence and
will it be deaths? In Egypt around 2000 BC, a writer compared fear of the Pharaoh with
fear of epidemics. The Old Testament of the Bible refers to several epidemics, including
one that affected the Philistines, purportedly as punishment for seizing the Ark of the
Covenant. The British Isles were hit by at least 49 epidemics between AD 526 and 1087.
Epidemics can reshape societies, affect the course of military events, and change the
balance of power among different groups of people. An epidemic in Athens in 430 BC
created chaos in the city and contributed to defeat in its war with Sparta. Among the
best known of all epidemics was the Black Death, an epidemic of bubonic plague that broke
out in Europe in AD 1347. By 1351 an estimated 25 to 50 percent of the people in Europe
had died from the disease. The Black Death depopulated once-flourishing cities, left
villages vacant, and caused a decline in cultivated land.
When Europeans began to explore the Americas in the 15th century, they carried along
pathogens unknown in the new lands. Smallpox and measles raced through native populations
with devastating results. For example, by 1568, only 50 years after Hernan Cortes first
reached Mexico, the population of central Mexico had fallen from about 17 million to
about 3 million. It is doubtful that Cortes could have conquered the Aztecs as easily as
he did had this disaster not befallen the Aztecs.
BStigma of Disease Fears of disease, often coupled with ignorance, have led to horrifying
treatment of the afflicted. Outbreaks of plague in Europe were often blamed on Jews, who
were beaten and driven from their homes. During an epidemic in 17th-century Italy, people
suspected of being carriers of the plague were tortured and burned alive. Through the
ages people with leprosy were often isolated in leper houses, forbidden to marry, and
forced to wear a distinctive cloak or shake a rattle to announce their presence.
Even in supposedly advanced cultures, the stigma of disease remains. In recent years,
people with AIDS have heard that their illness was God's punishment for immoral behavior.
Many have been ostracized by family, friends, and even physicians who are fearful of
contagion. People with AIDS have also been denied housing, medical treatment, and the
right to travel to foreign countries.
VIILIFE SPAN 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.
Robert Sikorski
Richard Peters
Human Disease. Microsoft? Encarta? Encyclopedia 2001. ? 1993-2000 Microsoft Corporation.
All rights reserved.