· Viruses are even smaller than bacteria and can

Bacteria are one-cell organisms that are
responsible for illnesses such as urinary tract infections, anthrax, and
tuberculosis. It is a gel-like matrix composed of water, enzymes, nutrients,
wastes, and gases and contains cell structures
such as ribosomes, a chromosome, and plasmids. The cell envelope encases the cytoplasm and all its components. Unlike the eukaryotic
cells, bacteria do not have a membrane-enclosed nucleus. Pathogenic bacteria cause infection by entering the body
through a wound, some disease-causing bacteria are carried in the mouth, nose,
throat and lower respiratory tract. They can spread when an animal comes into
direct contact with droplets when an infected animal coughs or sneezes, or
through direct contact with saliva or mucus on unwashed hands or surfaces. Most bacteria reproduce by binary
fission which is an asexual process where a bacterium divides into two
identical bacterial cells. These two new cells grow and then each divides to form two new cells, resulting
in a total of four cells with identical DNA from a single parent cell. When
conditions are favourable such as the
optimal temperature, moisture and nutrients are available, bacteria can grow
and reproduce at a rapid rate.

Viruses are even
smaller than bacteria and can cause a large quantity of diseases from a common
cold to AIDS. All viruses contain a nucleic acid genome and a protein capsid
that covers the genome which makes up the nucleocapsid. Many animal viruses also contain a lipid envelope.
A virus can be contracted through direct contact with an infected animal or its
sneeze or cough droplets. Viruses can also be spread from mother to young
during birth, colostrum or in the milk that the young suckles on. A
bacteriophage is a virus that infects and replicates with the bacterium in the
host animal. They replicate the bacterium when their genome is injected into
their cytoplasm. The viral replication cycle produces dramatic amounts of biochemical
and structural changes in the cells of the host, which can change the functions
of the cell or even destroy it. An example of a bacteriophage known to follow this
cycle is E. coli. Animal viruses do not have to penetrate a cell wall to gain
access to the host cell. Non-enveloped animal viruses have two different ways
in which they enter the cell wall.  When
a protein in the viral capsid binds to its receptor on the host cell, the virus
may be taken inside the cell by a cavity during the normal cell process of
receptor-mediated endocytosis. Non-enveloped viruses use an alternative method
where capsid proteins undergo shape changes when the capsid become bound to the
receptor, creating channels in the host cell membrane. The viral genome is then
injected into the host cell through these channels. Enveloped viruses also have
two ways of entering cells after binding to their receptors one of these
methods is by receptor-mediated endocytosis in a similar fashion to some
non-enveloped viruses. On the other hand, fusion only occurs with enveloped
virions. These viruses, which include HIV among others, use specialised fusion
proteins in their envelopes to cause the envelope to combine with the plasma
membrane of the cell, therefore releasing the viruses’ capsid and genome into
the cells cytoplasm.

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Fungi cause many skin diseases, such as athlete’s foot and ringworm; however other
types of fungi can infect your lungs or nervous system. Yeast dermatitis
can occur when the fungus candida infects sensitive
skin. Symptoms are hyperpigmentation, itching and
redness. Fungal cells are eukaryotes and therefore contain a membrane-bound
nucleus where the DNA is
wrapped around histone proteins, whilst a few types of fungi contain looped DNA.
Fungi reproduce by
spreading microscopic spores. These spores are often present in the air and
soil, where they can be inhaled or come into contact with the surfaces of the
body, primarily the skin.
Fungi reproduce by
fragmentation, budding, or producing spores which are forms of asexual
reproduction. New colonies can be grown by fragments of hyphae. Mycelial
fragmentation occurs when a fungal
mycelium separates into pieces with each component growing into a
separate mycelium. Somatic cells in
yeast form buds. The
major factors affecting the growth of
fungi are nutrients, temperature, light, aeration, pH, and water activity. Nutrient
requirements for fungi may vary; some fungi thrive on substrates with high
sugar or salt content.

Helminths. An organism which lives in or on another organism, its host, and
benefits by deriving nutrients at the other’s expense. Specifically, a helminth
is a parasitic worm, such as a tapeworm, flatworm, or nematode. Tapeworms are
segmented flatworms that effect animals such as cows, pigs, and humans. Tapeworms
attach to the insides of the intestines of animals such as cows, pigs, and humans.
They get food by eating the host’s partly digested food therefore depriving the host of nutrients. A tapeworm can reproduce sexually, either through
self-fertilization or cross-fertilization with another tapeworm, or asexually, by breaking
off proglottid (each segment in the strobila of a tapeworm, containing a
complete sexually mature reproductive system) segments at the end of the trunk.
Although parasites harm their hosts, it is in the parasite’s doesn’t want to kill
the host as it relies on
the host’s body and
bodily functions, such as digestion or blood circulation, to live. Helminths do
not have a set structure as they are individual organisms with their own
adapted characteristics.


Protozoa are microscopic, one-celled organisms that can be free-living or
parasitic in nature. They are able to multiply in humans, which contributes to
their survival and also permits serious infections to develop from just a
single organism. Transmission of protozoa that live in a human’s intestine to
another human typically occurs through a fecal-oral route, for example,
contaminated food or water or person-to-person contact. Protozoa that live in
the blood or tissue of humans are transmitted to other humans by an arthropod
vector, for example, through the bite of a mosquito or sand fly.  Amoebas
are members of the sarcodines. They may reproduce both asexually or sexually. Asexual reproduction is a simple cell
division, called fission, in which division of the genetic material can be seen
in the nucleus or centre of the cell. Entamoeba histolyticus is most often spread through the
ingestion of infected human feces. There are two species of Acanthamoeba that
are free-living: A. castellani and A. culbertsoni. These species can be found in
freshwater, saltwater, soil and sewage.
Entamoeba histolytica is usually an asymptomatic disease
(meaning symptoms aren’t obvious). Severe infections can cause colitis,
resulting in bloody diarrhea. Hematogenous spread (spread through the body via
the blood stream) causes damage to and failure of major organ systems. Symptoms
are dependent on the organ system involved but death is the usual outcome.




b) The immune


The immune
system is there to keep you healthy by attacking foreign bodies or cells
created in your body that threaten your health. When pathogens or infectious
agents such as bacteria, viruses, fungi and parasites attack your body the
immune system begins a series of innate and adaptive defences.

of the immune system

Skin – This is
one of the most important parts of the immune system because it is the body’s
first defence against foreign bodies and infectious agents. When the skin is
broken, due to a wound or cracked dry skin, foreign bodies can then entre the

tonsils and thymus
– these organs produce antibodies such as lymphocytes and T lymphocytes.

lymph nodes and vessels – These make up the lymphatic system which is
an important part of the immune system as the lymph nodes filter lymph fluid as
it flows through them which traps foreign bodies, which are then destroyed by
lymphocytes. The network of lymph nodes and vessels throughout the body carry
lymph fluid (nutrients) and waste material between the bloodstream and body

– this organ filters the blood by removing old or damaged blood cells and
platelets and helps the animal’s immune system by destroying foreign bodies.

– Soft tissue found mostly inside long bones of the arms and legs, vertebrae
and pelvic bones. Bone marrow is made up of red marrow, which produces red and
white blood cells and platelets (important for coagulation), and yellow marrow
which contains stored fat and connective tissue and produces some white blood

blood cells
– These blood cells are made in the bone marrow and protect the body against
infection. If an infection develops, white blood cells attack and destroy the
foreign body causing the infection.

of causes and symptoms of disease

is the physiological, pathological, or biochemical mechanism that results in
the development of disease. In simpler terms, it is what caused the disease,
such as tissue breakdown. This is different from pathophysiology, or
physiological responses which simply are the symptoms of an infection or
disease that you display when your body is fighting the foreign bodies.
Therefore the physiological responses happen after the original pathogenesis.

of immune systems

– This is non-specific immune responses that are naturally present from birth.
These are responses from inflammation to an increase in mucus production.
Innate leukocytes include: eosinophils, basophils, natural killer cells, and
mast cells.

– This is a specific, acquired immune system that involves highly specialised
cells and processes that eliminate or prevent the growth of pathogens. Adaptive
immune systems are ‘learnt’ by the body. Once it has fought off a pathogen
using a specialised system the body remembers the process and cells needed, and
then when the same or a similar pathogen attacks the body knows how to react.

immunity –
Naturally acquired
active immunity occurs
when a person is exposed to a live pathogen, and develops a primary immune response, which leads to
immunological memory. This type of immunity is
natural because it is not
induced by deliberate exposure.

immunity –
Artificially acquired
active immunity can be
induced by a vaccine, a substance that contains antigen. A vaccine stimulates a
primary response against the antigen without causing symptoms of the disease.

and negatives of immune responses.

The immune
system is able to create strong and efficient immune responses against
pathogens and parasites without damaging organs in the body. This is mainly due
to the contraction and destruction of the immune response after the infectious
agent has been controlled. During contraction the majority of T cells are
destroyed but the ones left over survive as memory cells to fight a similar or
the same pathogen that infects the animal. However a negative of the general
immune system is that pathogens are still able to affect our body multiple times,
like the common cold.

specifically innate immunity is effective because it means we have something to
defend the body from birth. Without this most offspring would die within days. Although
innate immunity is still non-specific and in some cases won’t do anything to
help the body fight the particular pathogen.

On the other
hand adaptive immunity is specific, meaning it can efficiently and effectively
target and terminate pathogens, however these responses must be learnt, which
means the animal would have to have already been exposed to the pathogen. This
could be by vaccination, which is close to harmless (unless you have an
allergic reaction), or by previously being attacked by the pathogen which could
cause irreparable damage to the animals body


Bacterial infection

When foreign
bacteria enter the body they are at first undetected until a stable population
is reached. At this point they change their behaviour and start damaging the
body by altering the environment around them. This causes the immune system to
become aware of the invasion and activate the macrophages.

macrophages ingest and destroy the bacteria cells by phagocytosis and also help
to control inflammation by ordering blood vessels to release water in the
infected area so fighting of the infection in that area is easier. When the
macrophages have destroyed all they can they release message proteins that will
communicate urgency in that location. Upon activation, neutrophils leave their
usual patrol route in the blood and move to the site of infection, where they
create barriers that trap and kill bacteria. However during the process of
killing bacteria, they also attack and kill healthy host cells. As they are so
strong they have evolves to commit suicide after 5 days so that they do not
cause the host too much damage.

If the
infection hasn’t yet been neutralised, dendritic cells get activated. These
cells collect dead bacteria, rip them to pieces, and present the parts on their
outer layer. The dendrite cells then travel to the closest lymph node, which
will take around a day, where multiple helper and killer T cells are waiting to
be activated. The dendrites then look for T cells with a specialised set up to
combine with the shreds of the bacteria cells that are presented on the
dendrites membrane. As soon as one is found a chemical reaction takes place
which activates the T cell. This causes the T cell to rapidly duplicate. Some
of these cells remain in the lymph nodes as memory cells, allowing for relative
immunity if the bacteria attacks again. Others will travel to the site of
infection to help fight off the bacteria, whilst some travel to the centre of
the lymph nodes to activate the B cells which then duplicate rapidly and
produces many antibodies to also help fight the infection. B cells produce so
many antibodies that they are at a high risk of denaturing, therefore a helper T
cell stimulates the B cell to prevent it from dying out from exhaustion whilst
the infection is still active. Once the infection is neutralised the B cells
will die to prevent the body from wasting unnecessary energy.

antibodies produced by the B cells are small proteins designed to bind to the
surface of the intruder. During an infection, millions of antibodies flood the
blood and saturate the body until they reach the site of infection, where they
disable bacteria which render them useless or kill them in the process. They
also stun the bacteria, making them an easy target for macrophages to ingest
and destroy.

Once the
infection is neutralised all of the host cells that were destroyed are rapidly
replaced and most immune cells that are no longer needed destroy themselves.
This is effective because it means energy and resources are not wasted on
things like B and helper T cells. However the bacteria still goes undetected
when it first enters the body, until it has reproduced to the point of damaging
the body’s cells. If the bacteria were detected sooner it wouldn’t be able to
reproduce enough to cause any significant damage.




destroy germs as soon as they detect them. However, if a viral infection begins
to take hold we fight back using a stronger defence of white blood cells, T and
B lymphocytes. Special proteins made by the B cells bind to the virus to stop
them replicating, and also tag them so that macrophages know to destroy them by
phagocytosis. For T cells, some guard the body and raise the alarm when they
detect invading viruses; others kill virus-infected cells directly, or help B
cells to produce antibodies. Once the virus has been neutralised, a small
amount of the specialised B and T cells remain to retain a memory of the virus,
to combat more efficiently if it occurs again. This is effective because it is
only using two types of bodily cells that work together efficiently and get to
work as soon as a virus is detected. Also, storing memory cells, once the infection
is over, means that if the virus returns it will be destroyed even faster and
more efficiently than the last. On the other hand, the fact that fighting off
the virus is left down to two types of cells means that if the virus does over
power either the T or B lymphocytes there isn’t many other walls of defence



When the
fungi enter the body they are recognised by cells of the innate immune system,
macrophages and dendritic cells, which bind to part of fungal cell walls using
pattern recognition receptors on their surface – C-type lectin and toll-like receptors
are the most important in this instance. Once bound to the fungi, the PRR’s
signal using their intracellular tails or associated molecules, which results
in phagocytosis, the initiation of killing mechanisms such as; the production
of reactive oxygen species, and also create memory cells to help with future,
similar infections. This is effective because it initially uses cells from the
innate immune system which means they will come into action almost immediately,
instead of relying on the adaptive system to specialise itself to start fighting
the pathogen. However it is ineffective because in order to actually affect the
virus it needs cells from the adaptive immune system, such as the c-type lectin
and toll-like receptors, which will take longer to kick into action.


– Helminths

shows that parasitic worms have the ability to deactivate certain cells of the
immune system, which means a weakened immune response. This is good for the
parasite and the host as it decreases the immune response against harmless
allergens that the worm carries, gut flora, and the hosts own cells. The immune
response is effective in the sense that it’s not causing unnecessary damage to
the hosts body but it is not greatly effecting the parasite itself because the
immune response is stunted.