Health Studies Lecture - 2.docx
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Plagues and People
Parasites – organism that grows, feeds, and is sheltered on or in a different organism
and that does not contribute to the survival of its host. Sheltered inside the host, uses
the host’s resources to survive and replicate, but does not provide any resources to help
the host survive.
o (someone lives in your house, uses all your resources (food, shelter) but does
not pay rent
o They are not able to survive on their own so they use the host for nourishment
and replication. Parasites can be in different forms.
Virus – ultimate micro-parasite – smaller than bacteria; neither cells not organisms; can
only reproduce within their host. It’s a collection of genetic material that is engulfed in a
protein. They can only reproduce within host. They can only be seen in an electron
microscope. Examples of the smallest micro parasites.
Parasites that are cells: bacterium’s, and protozoa’s (unicellular) one celled parasites.
Are usually seen with a light microscope.
Macro-parasites – composed of many cells; cycles through transmission stages (eggs
and larvae) which pass into the external environment
Usually don’t multiply within the host , but they mostly use the host for multiplication
but also to cycle through transmission stages. Use the host to lay eggs for instance.
Transmission stages of eggs and larvae, which transmits its way into the external
environment and find its way to impact the host once again.
Are larger , and can be seen using the naked eye. An example of a macro parasite is the
plasmodium . Which is the parasite that causes malaria? Flies and ticks are larger and
are able to cause harm to humans. Macro parasites can live on surfaces but they can
also live inside the human body.
HIV virus lives inside the host (human body) . Some of them have complex and simple
Malaria is an example where the main host is humans.
For a disease to occur transmission has to happen, and the disease has to move from
one host to another.
Transmission - movement of a parasite from host to host
Incubation period: interval of time required for development of a disease in a host. Time
needed for the disease to develop inside the host. The parasite or the pathogen is
replicating or doing something. You do not see the symptoms of the disease.
The person doesn’t feel or look sick, but replication is occurring inside the body, and
symptoms will be seen later.
Latent Period: a period of time, where a person or host will be exposed to a pathogen,
and there is nothing happening in between and after which the person experiences
symptoms of the disease. The difference between the two is that, within incubation
period, there is a period of activity that is occurring.
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Latent period- seemingly inactive period between exposure to an infection and
Syphyllis is an example of a latent disease. We cannot see air pollutants that exist within
our bodies. The latent period is very disease specific. Different factors that trigger a
disease occur due to the influences of the external environment, the host, etc.
For the latent period, there is a seemingly inactive period between exposure to an
infection and subsequent illness. The host has been exposed but there is nothing inside
the host that is happening that shows any type of activity. The parasite is in a dormant
Incubation period – period of activity that can be seen
Latent period: the parasite , or pathogen is in a dormant stage
Parasite virulence: how strong or capable a parasite is in inflicting harm on its host. Is it
able to cause disease , or is it weak ?
Zoonotic infections: are infections that originate from animal sources. Animal infections
that get transmitted to human hosts.
What are the main factors that influence the occurrence of a disease?
- Epidemiological triad
- Host , environment and an agent have to be present to cause a disease
- Host: immunity (susceptible) , genetics that implicate whether they are going to get
- Environment: that promotes exposure such as contaminated air, water,
- Agent: biological , physical , chemical and psychosocial- stress, anxiety, rate of growth
and persistence, virulence- how strong or weak is it to cause harm. All of these have to
be present in a certain situation.
- Person (host) , environment ( contaminated water) and the agent (bacteria inside the
Modes of Disease Transmission
- Direct transmission- from person to person
- Indirect transmission- through a common route/vector, Eg. Contaminated water /air,
mosquito. West nil virus or malaria (mosquito moves parasite from one person to
another) . the mosquito is not the source of infection but a vector that is used to
transmit the infection.
Portal of entry:
- Dermal – through the skin, e.g. fungus
- Ingestion – through the mouth, e.g. E.coli in water, contaminated food
- Inhalation – during respiration, e.g. particulate matter
- Aspiration – airway entry (legionnaires disease)
Types of Diseases
Broad spectrum of disease severity – Iceberg concept
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