|
The
Rise of Contagious Disease
The rise of contagious disease in humans
In prehistoric times, epidemics of infectious
diseases in humans were almost certainly rare for a number of
reasons. Humans lived
in hunter-gatherer communities, and this life style required large
areas of land to collect food from. Each
small group of people lived an isolated existence away from other
populations, preventing the spread of disease. There
were no domesticated animals kept in close proximity to humans, and
thus, they could not serve as reservoirs of pathogens or
intermediaries of disease transmission.
The domestication of animals and the
cultivation of plants by early humans brought about a shift from a
hunter-gatherer, nomadic existence to an agricultural society that
could now stay in one place. This
brought about several changes that facilitated the spread of
infectious diseases. Agriculture
demanded that humans now live in larger groups to help in the
planting, care and harvesting of crops. Keeping
animals meant living near them and farmers would often sleep under
the same roof as their animals, so transmission of infectious agents
between humans and animals was now possible. Although
the availability of food was higher than in the past, malnutrition
was still a problem because of poorer, mostly carbohydrate, diets. (In
contrast, the diet of the hunter-gatherer was healthier, including
more meat.) This led to
weaker immune systems. The
abundance of food also allowed segments of the population to pursue
occupations outside the realm of agriculture. Many
of these pursuits involved interaction with others, fostering the
growth of large cities and intercity commerce. High
densities of people encouraged the rapid spread of disease, and this
was exacerbated by poor public hygiene. Since
the development of agriculture, infectious disease has increasingly
plagued humans.
As diseases became more common, various
efforts were made to attempt to treat these maladies. Traditional
societies developed a variety of herbal and animal medicines and
often a medical caste to administer them. There
is a prejudice among many people now that these treatments must have
some utility or the traditional societies would not have continued
them. Doubtless, some
of these had some efficacy, but others certainly did not. Why
might non-efficacious remedies persist? A
primary reason is that the largely uneducated populations had little
choice but to use them, as there was essentially no alternative. Secondly,
even the most useless compound did seem to work at least some
of the time. Partly, this is because people simply recover by themselves
some of the time, but such recoveries were certainly attributed to
the treatment. Another
powerful reason is the placebo effect, in which optimism of a
patient about the treatment really does have a helpful effect. Quite
possibly, this is a result of neural effects on the immune system,
but the mechanisms are unclear.
In more developed societies, doctors and
chemists concocted elixirs and powders of suspicious content and
declared them as cures for all sorts of illnesses, though it was
absolutely clear that these treatments were harmful to the patient
and could not possibly provide a cure. Even
in rather recent times, the application of leeches and blood letting
was a popular treatment for almost any complaint. It
was thought the leeches removed bad blood from the patient and cured
them. As odd as it may
seem, electrical treatments were also attempted in the early 20th
century. As above,
normal patient recovery and the placebo effect yielded some
apparently positive results. Nevertheless,
the egregious marketing of patent medicines, with extravagant claims
that bordered on the humorous, gradually made the general public
skeptical of most medical treatments by the early 20th
century.
The true nature of infectious diseases was not
understood. However, it
had been obvious to many for some time that being in the presence of
someone who had certain illnesses would increase their likelihood of
coming down with the illness. This
explains the flight from many of the cities of Europe during the
epidemic of bubonic plague in the 14th century.
In the last 100 years however, human society
has made amazing progress in the battle against infections diseases.
At the turn of the
century, the top killers were pneumonia, influenza, tuberculosis and
diarrhea. Infectious
disease accounted for more than 33% of all deaths. In
1998, the leading causes of death were heart disease, cancer and
stroke, with infectious disease accounting for less than 4% of all
deaths. As we have come to understand the nature of disease and the
pathogens that cause it, we have limited the impact of each illness.
Five major advances have helped to decrease
the incidence of infectious disease in humans: quarantine of sick
individuals, better sanitation of human waste, pest control,
vaccines, and antibiotics.
Quarantine
One of the first methods of attacking
infectious disease was the isolation of contagious individuals to
prevent them from spreading the disease to others. The
practice of quarantine began during the 14th
century in an effort to protect coastal cities from plague
epidemics. Ships
arriving in Venice from infected ports were required to sit at
anchor for 40 days before entering. The
word quarantine is derived from the Latin word quadraginta, meaning
forty.
Quarantine can take several forms. The
most common form of quarantine is the isolation of sick individuals
to a limited access location. This
prevents them from spreading the illness to others during the course
of a disease. Recent outbreaks caused by Ebola virus in Africa have been
rapidly halted by the immediate use of quarantine practices,
limiting the number of deaths. Quarantine
can also take the form of restricting the movement of healthy
populations during epidemics and isolating healthy individuals
exposed to an illness.
Early in history of its use, quarantine was
employed by local governments, but as populations became more mobile
and widespread epidemics became possible, quarantine activities were
assumed by national governments. A
number of yellow fever epidemics prompted the United States Congress
to pass Federal Quarantine Legislation in 1878 and many more acts
followed to empower the government to help prevent the spread of
disease.
At its best, quarantine is only a partially
effective means of slowing the spread of disease. With many diseases, an infected individual will be contagious
before showing any outward signs of illness. Carriers, who are asymptomatic people harboring the infectious
agent and capable of spreading it to others, also exist for many
infectious diseases. Having
them present in a population will allow continued new cases of the
illness. The previous
example of keeping ships at dock for a period of time would
sometimes fail, because rats carrying the fleas that spread the
plague-causing bacteria would swim ashore. Despite
these problems with quarantine, its practice still helps decrease
the number of individuals who come down with a disease.
The advent of effective methods for curing
infectious disease has greatly decreased the use of quarantine as a
method of disease prevention, but several diseases, including
cholera, diphtheria, tuberculosis, plague, yellow fever and viral
hemorrhagic fevers caused by Marburg, Ebola and Congo-Crimean
viruses are still candidates for quarantine. The
CDC monitors the incidence of these
diseases in other parts of the world and pays particular attention
to individuals entering this country from endemic areas.
Sanitation
is another major deterrent to the spread of disease and has been
practiced in various forms for centuries. Archeological
evidence shows the Etruscans of Italy (800-400 B.C.) and the Incas
of South America (1100-1500 A.D.) thought clean water and efficient
waste disposal were important. These
societies did it because they thought it would please the gods. The
Romans and Greeks realized sanitation was important for healthy
living and built latrines and sewer systems to isolate and remove
waste from living areas.
The connection between poor sanitation and the
spread of disease became clear through the studies of John Snow in
London. An outbreak of
cholera in the city of London was devastating the populace. By
carefully plotting the residence of each patient, Snow discovered
that there was a significant cluster of cholera patients around the
Broad Street water pump. Removal
of the pump handle stopped the epidemic. Snow
correctly concluded that germs in the water were causing cholera and
that polluted water was the means of transmitting the disease from
person to person. By
the late 19th century, the activism of reformers like Sir
Edwin Chadwick resulted in English laws regulating sanitation, which
significantly increased life expectancy.
Today, public water supplies in most areas are
treated to remove any pathogenic organisms that might be present. This
purification normally takes the form of settling tanks, filtration
and chlorine treatments. Most people would not consider this as a disease treatment,
but it really is. The
use of clean potable water and the treatment of waste water before
release, prevents the transmission of illness by waterways. Times
of war or natural disaster are often accompanied by outbreaks of
water-borne disease because of disruption of water and sewage
treatment, which illustrates how important sanitation is to modern
society.
Pest control
At the end of the 19th century, it
was realized that insect vectors were spreading several diseases. During
the Spanish-American War, the United States Army lost 958 soldiers
in battle, but over 5,000 to yellow fever. It
became clear that if the U.S. was to occupy Cuba until a stable
government could be formed, they would need to protect the troops
from yellow fever. Major
Walter Reed was appointed to head a commission whose charge was to
study the cause and transmission of the disease. After much work and
even the performance of transmission experiments upon themselves,
they showed that the common Cuban mosquito, Aedes aegypti,
spread yellow fever. Further experiments by other scientists later demonstrated
that the causative agent was a virus.
The findings of the Reed commission were
immediately put into practice. The city of Havana was divided into
20 districts with teams of sanitation officers overseeing each
sector. Mosquito
breeding grounds were the major targets. Stagnant
bodies of water were covered with a layer of oil and the people of
the city were told to cover any open barrels of water either with
screens or oil to prevent the mosquitoes from laying their eggs. Noncompliance
met with a fine of ten dollars, a large sum at that time. Their
efforts were a success with fewer than twenty people dying of yellow
fever in 1901, where thousands had died in previous years.
Vaccines
Immunization has allowed
the complete eradication of smallpox worldwide and eliminated
poliomyelitis from the Western hemisphere. It
has also been the cause of major reductions in the incidence of
other infectious diseases in the United States. Vaccines
are one of our most effective weapons against infectious diseases
and new vaccines are being continually developed that protect both
children and adults from potentially fatal illnesses. In
the 20th century, infant mortality rates in the United
States decreased by more than 90%, and much of this decline is
attributable to the introduction of routine vaccination of children.
Vaccines were first
discovered in the 18th century and have become more
important for disease prevention since then.
Vaccines are antigens
prepared from pathogens that can raise a protective immune response,
yet do not cause illness. These
prepared antigens will stimulate both B cells and T cells and help
to create memory cells that can later mount a vigorous immune
response to an encounter with the real pathogen. As
scientists have understood more about the pathogens that cause
infectious diseases and how our immune systems recognize and destroy
them, they have been able to identify the important antigens that
our bodies react to. This
has allowed the development of vaccines that contain only those
components that raise a protective immune response. Component
vaccines are more desirable than whole-cell vaccines, because the
body is only exposed to a few antigens, instead of the entire
pathogen. Advances in
microbiology and the pharmaceutical industry have made it easier to
prepare component vaccines and many of the vaccines used today are
of this type.
Antimicrobial
compounds
The discovery of antibiotics by Alexander Fleming in 1929 and
Gerhard Domagk怸s discovery of the broad antimicrobial activity of
the sulfonamides hinted at a new method for treating infectious
diseases. World War II
spurred further development of penicillin by Florey and Chain who
isolated and purified the compound. They
then demonstrated that injection of penicillin into infected mice
not only cured the disease, but also had very low toxicity to the
animal. The use of
penicillin in the war greatly reduced the number of casualties due
to wound infections. In
the ensuing years, an intense search for similar compounds with low
toxicity and high antimicrobial activity resulted in the isolation
of a large number of antimicrobial compounds, which ushered
biomedical science into the age of antibiotic chemotherapy. Chemotherapeutic
agents are chemically synthesized compounds that have antimicrobial
activity. Antibiotics
achieve their magic by having a selective toxicity. They
attack a process in the microorganism that has no counterpart in
humans. An antibiotic
can be broad spectrum, affecting a wide range of organisms, or
narrow spectrum, affecting only a small subset of microorganisms. A
clinically useful antibiotic should have as many as possible of the
following properties:
Selective toxicity -
The drug must be
detrimental the microorganism, yet do little harm to the host.
Wide spectrum of activity
-
The antibiotic should inhibit as many different types of bacteria
as possible.
Non-allergenic -
The induction of an
allergic reaction in the host makes an antibiotic no longer useful.
Permeable -
An effective antibiotic
must be able to reach the part of the body where the infection is
occurring.
Inexpensive to produce
Chemically stable -
It must have a reasonable
stability both on the shelf and inside the body.
Difficult for the microbe to develop a resistance to
From
Microbiology and Bacteriology: The world of microbes
http://www.bact.wisc.edu/Microtextbook/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=294&page=1
|