General Introduction:
hunter/gatherers versus agriculturalists,
The rise of agriculture contributed to the spread of disease.
Tuberculosis and diarrheal disease arose with the advent of farming, measles
and bubonic plague appeared with the advent of large cities, which were
made possible by agricultural practices. Population densities of
hunter-gatherers are rarely over one person per ten square miles, while
farmers average 100 times that. This crowding facilitates the spreading of
pathogenic diseases. The domestication of animals also facilitates easier
spreading of diseases from animals to humans. Farmers also produce a few
starch rich crops that are lacking in essential vitamins, minerals and amino
acids, populations that rely on a lack of diverse foods have lower quality
health. Skeletons from farmer societies also showed signs of anemia,
tuberculosis, leprosy, enamel defects and other diseases tied to malnutrition.
The average height of some societies also drastically decreased between the
end of ice ages to the classical era.In the context of disease hunter gatherer
societies are advantageous to agricultural societies as they practice a lifestyle
of diverse diet that permits better nutrition, a low population density and
domestic animal population that reduces the spread of parasites and
infectious disease.
predation and parasitism,
Predation:
Predation refers to the act of killing another animal for food.example, Lions
eating zebras or zebras eating grass. Predators are well adapted to finding,
overpowering and quickly killing prey. Example, a tigers strength, speed,
camouflage and stalking behavior. Prey are well adapted at hiding, hindering
and running from predators. For example, a caterpillar’s camouflage, torns
and toxicity. A predator should also have immunity against poisons of a prey.
Parasitism:
refers to the act in which one organism lives in or on another organism as a
parasite.Parasites grow, feed and shelter inside or upon a host organism, the
relationship is harmful to the host organism. Though most parasites do not kill
the host organism, some parasites may eventually kill the host. These
parasites are pathogenic, causing diseases in the host. Ectoparasites such as
lice, mosquitoes and fleas, protozoans such as amoeba and Plasmodium, and
worms such as roundworm, tapeworm, and pinworm are parasitic in humans.
Aphids and some insects are plant parasites drinking the sap.
The parasites in the intestine of animals feed on partly digested food.
Generally, parasites are smaller in size than the host organism. But, they
exhibit higher reproduction rates once they invade the host. Some of the
developmental stages of the parasitic life cycle occur inside the host.
Predation and parasitism are two interspecific relationships that occur
between two different species of an ecosystem. In predation, the predator
immediately kills the prey while in parasitism, the parasite does not kill the
host organism. Generally, a predator is large in size than the prey. On the
other hand, a parasite is smaller than the host organism. The main difference
between predation and parasitism is the type of relationship between the
individuals of the two species.
size of pathogens
Flatworm animal length (maximum) 25 m
Loa loa
Loa loa
length (female)
20-70 mm
Tongue-eating louse
length (female)
8-29 mm
Pinworm
8-13 mm
Head louse length 2.5-3 mm
Tapeworm length (minimum) 1 mm
Scabies
length (female)
Giardia
length10-20 μm
0.3–0.45 mm
Yeast infection
diameter
Lyme disease
length10 μm
Malaria
7-14 μm
Plague
lengtha few μm
Tuberculosis
10-12 μm
Yersinia
length2-4 μm
PlasmodiumMalaria
diameter (asexual form) 1-2 μm
Mycoplasma genitalium (one cause of urethritis) length600 nm
HIV
diameter
120 nm
Orthomyxoviridae Influenza
diameter
Virus Rhinovirus Common cold
Porcine circovirus diameter
50-120 nm
30 nm
17 nm
Constructs of disease prior to ‘germ theory’
The miasma theory
the predominant theory of disease transmission before the germ theory. It held
that diseases such as cholera and the Black Death were caused by a miasma
a noxious form of "bad air" emanating from rotting organic matter.Miasma was
considered to be a poisonous vapor filled with particles from decomposed
matter that was identifiable by a foul smell. The theory was that diseases were
the product of environmental factors such as contaminated water, bad
smelling air and poor hygiene. Infections were not passed between individuals
but would affect those within a locale that gave rise to such vapors.
Evil Eye and Witchcraft
People often believed the existence of supernatural forces or spirits to explain
events that do not have a clear biological or psychological explanation, a
tendency that is especially acute for harmful events. In ancient times, people
needed a theory for predicting, and from which they could attempt to control,
the spread of disease. A belief in contagious and contaminating evil forces
would have provided a functionally equivalent framework for prediction and
management, identifying both the infection and transmission profile of
pathogens. Some people also believed disease was caused by magic and
witchcraft throughout history, a case in point is the escalation of witch-hunts in
response to the Black Death.
These incorrect assumptions affected overall health negatively in some cases
as the case of witch trials or people congregating to pray, but positive in some
cases as people practiced proper hygiene and worked towards removing
rotting organic matter and sources of foul smells which may be indicative of
harmful bacteria.
Discussion of disease causing organisms, vectors and transmissions.
Vector-borne diseases are human illnesses caused by parasites,
viruses and bacteria that are transmitted by vectors. Such diseases include
malaria, dengue, schistosomiasis, human African trypanosomiasis,
leishmaniasis, Chagas disease, yellow fever, Japanese encephalitis and
onchocerciasis.
Vectors are living organisms that can transmit infectious pathogens
between humans, or from animals to humans. Many of these vectors are
bloodsucking insects, which ingest disease-producing microorganisms during
a blood meal from an infected host (human or animal) and later transmit it into
a new host, after the pathogen has replicated.
Type 1, 2 and 3 epidemics
Type 1 epidemic is a common source outbreak. These occur when the rise in
cases of an infection occurs after a group of people all come into contact with
the same unsafe source of infection (the common source), such as
contaminated food or water. Example a village drinking from a well with water
contaminated by a dead animal causes everyone in the village to fall ill. The
infected water is the common source.
Type 2 epidemic is a propagated or progressive epidemic. These occur when
the infection spreads from person to person. The infectious agents causing
the disease pass from one host to another, either directly from person to
person, or indirectly via vectors, or another medium such as water. The
distribution of malaria cases is a good example of a propagated epidemic,
because increased numbers of malaria cases occur again and again at
different times.
A type 3 epidemic is a mixed epidemic. These show characteristics of both
common source and propagated epidemics. So a mixed epidemic can start
with a common source and be followed by a propagated spread.
An example is an outbreak of typhoid fever from a common contaminated
source such as a well polluted by feaces. The fever can then spread from
person to person thus exhibiting characteristics of common source and
propagated outbreak.
Six plagues of the ancient world
1. Pharaoh's Plague
A researcher in Egypt unearthed cat and human fleas ,known to be bubonic
plague carriers in some cases, in and around the homes of an ancient
Egyptian excavation site . Another clue that the Pharaoh's plague was the
bubonic plague is that previous excavations along the Nile Delta had turned
up Nile rats, an endemic species, dating to the 16th and 17th century B.C. The
plague's main carrier flea is thought to be native to the Nile Valley and is
known to be a Nile rat parasite.The Nile provided an ideal spot for rats to carry
the plague into urban communities. Around 3500 B.C., people began to build
cities next to the Nile. During floods, the habitat of the Nile rat was disturbed,
sending the rodent and its fleas with the plague bacteria into the human
settlements.
2. Plague of Athens
The Plague of Athens was an epidemic that devastated the city-state of
Athens in ancient Greece in 430 BC. The plague killed an estimated
75,000 to 100,000 people, around one quarter of the population, and is
believed to have entered Athens through Piraeus, the city's port and
sole source of food and supplies. It is believed that the epidemic was an
outbreak of Typhus due to the consistency in the mortality rate and DNA
evidence found in an ancient Greek burial pit.
3. Cyprian Plague
The Plague of Cyprian was a pandemic that afflicted the Roman Empire
about from AD 249 to 262. The agent of the plague is highly speculative
because of sparse sourcing, but suspects have included smallpox,
pandemic influenza, and viral hemorrhagic fever (filoviruses) like the
Ebola virus. The plague is thought to have caused widespread
manpower shortages for food production and the Roman army, severely
weakening the empire
4. Roman Plague
The Antonine Plague of 165 to 180 AD, also known as the Plague of
Galen, was the first known pandemic impacting the Roman Empire,
possibly contracted and spread by soldiers who were returning from
campaigns in the Near East. Scholars generally believe the plague was
smallpox
5. Plague of Justinian
The Plague of Justinian or Justinianic Plague was the first major
outbreak of the first plague pandemic. The disease afflicted the entire
Mediterranean Basin, Europe, and the Near East, severely affecting the
Sasanian Empire and the Byzantine Empire and especially its capital,
Constantinople. In Constantinople it was killing 10,000 people a day at
its height. Researchers confirmed earlier speculation that the cause of
the Plague of Justinian was Yersinia pestis, the same bacterium
responsible for the Black Death. The plague's long-term effects on
European and Christian history were significant. As the disease spread
to port cities, the struggling Goths were able to carry out successful
campaigns against the Byzantines. The plague weakened the Byzantine
Empire to a critical point, when Byzantine armies had nearly retaken all
of Italy and reunited the core of the Western Roman Empire with the
Eastern Roman Empire, the Lombards invaded Northern Italy, defeated
the small Byzantine army that had been left behind and established the
Kingdom of the Lombards which went on to become the kingdom of
Italy. The Roman empire would remain split for the rest of history until
their total defeat by Germanic tribes in the west and Turks in the east.
Bubonic Plague
Bubonic plague is an infection spread mostly to humans by infected
fleas that travel on rodents. Called the Black Death, it killed millions of
Europeans during the Middle Ages. Prevention includes reducing your
exposure to mice, rats, squirrels and other animals that may be
infected. There is no vaccine. The Justinian plague mentioned above
was the first instance of the bubonic plague. Later on in the middle ages
the infamous black death would emerge and become the deadliest
disease outbreak in history.It killed one third of the European human
population.Historians believe that society subsequently became more
violent as the mass mortality rate cheapened life and thus increased
warfare, crime, revolt, and persecution. Europe lost a majority of its
doctors and many villages were left entirely abandoned.