· Viruses are even smaller than bacteria and can

·        Bacteria are one-cell organisms that areresponsible for illnesses such as urinary tract infections, anthrax, andtuberculosis. It is a gel-like matrix composed of water, enzymes, nutrients,wastes, and gases and contains cell structuressuch as ribosomes, a chromosome, and plasmids. The cell envelope encases the cytoplasm and all its components. Unlike the eukaryoticcells, bacteria do not have a membrane-enclosed nucleus. Pathogenic bacteria cause infection by entering the bodythrough a wound, some disease-causing bacteria are carried in the mouth, nose,throat and lower respiratory tract. They can spread when an animal comes intodirect contact with droplets when an infected animal coughs or sneezes, orthrough direct contact with saliva or mucus on unwashed hands or surfaces. Most bacteria reproduce by binaryfission which is an asexual process where a bacterium divides into twoidentical bacterial cells.

These two new cells grow and then each divides to form two new cells, resultingin a total of four cells with identical DNA from a single parent cell. Whenconditions are favourable such as theoptimal temperature, moisture and nutrients are available, bacteria can growand reproduce at a rapid rate.·        Viruses are evensmaller than bacteria and can cause a large quantity of diseases from a commoncold to AIDS. All viruses contain a nucleic acid genome and a protein capsidthat 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 itssneeze or cough droplets. Viruses can also be spread from mother to youngduring birth, colostrum or in the milk that the young suckles on.

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Abacteriophage is a virus that infects and replicates with the bacterium in thehost animal. They replicate the bacterium when their genome is injected intotheir cytoplasm. The viral replication cycle produces dramatic amounts of biochemicaland structural changes in the cells of the host, which can change the functionsof the cell or even destroy it. An example of a bacteriophage known to follow thiscycle is E. coli.

Animal viruses do not have to penetrate a cell wall to gainaccess to the host cell. Non-enveloped animal viruses have two different waysin which they enter the cell wall.  Whena protein in the viral capsid binds to its receptor on the host cell, the virusmay be taken inside the cell by a cavity during the normal cell process ofreceptor-mediated endocytosis. Non-enveloped viruses use an alternative methodwhere capsid proteins undergo shape changes when the capsid become bound to thereceptor, creating channels in the host cell membrane. The viral genome is theninjected into the host cell through these channels. Enveloped viruses also havetwo ways of entering cells after binding to their receptors one of thesemethods is by receptor-mediated endocytosis in a similar fashion to somenon-enveloped viruses. On the other hand, fusion only occurs with envelopedvirions. These viruses, which include HIV among others, use specialised fusionproteins in their envelopes to cause the envelope to combine with the plasmamembrane of the cell, therefore releasing the viruses’ capsid and genome intothe cells cytoplasm.

·        Fungi cause many skin diseases, such as athlete’s foot and ringworm; however othertypes of fungi can infect your lungs or nervous system. Yeast dermatitiscan occur when the fungus candida infects sensitiveskin. Symptoms are hyperpigmentation, itching andredness.

Fungal cells are eukaryotes and therefore contain a membrane-boundnucleus where the DNA iswrapped around histone proteins, whilst a few types of fungi contain looped DNA.Fungi reproduce byspreading microscopic spores. These spores are often present in the air andsoil, where they can be inhaled or come into contact with the surfaces of thebody, primarily the skin.

Fungi reproduce byfragmentation, budding, or producing spores which are forms of asexualreproduction. New colonies can be grown by fragments of hyphae. Mycelialfragmentation occurs when a fungalmycelium separates into pieces with each component growing into aseparate mycelium. Somatic cells inyeast form buds. Themajor factors affecting the growth offungi are nutrients, temperature, light, aeration, pH, and water activity. Nutrientrequirements for fungi may vary; some fungi thrive on substrates with highsugar or salt content.·        Helminths. An organism which lives in or on another organism, its host, andbenefits by deriving nutrients at the other’s expense.

Specifically, a helminthis a parasitic worm, such as a tapeworm, flatworm, or nematode. Tapeworms aresegmented flatworms that effect animals such as cows, pigs, and humans. Tapewormsattach 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 throughself-fertilization or cross-fertilization with another tapeworm, or asexually, by breakingoff proglottid (each segment in the strobila of a tapeworm, containing acomplete 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 killthe host as it relies onthe host’s body andbodily functions, such as digestion or blood circulation, to live. Helminths donot have a set structure as they are individual organisms with their ownadapted characteristics.

 ·        Protozoa are microscopic, one-celled organisms that can be free-living orparasitic in nature. They are able to multiply in humans, which contributes totheir survival and also permits serious infections to develop from just asingle organism. Transmission of protozoa that live in a human’s intestine toanother human typically occurs through a fecal-oral route, for example,contaminated food or water or person-to-person contact. Protozoa that live inthe blood or tissue of humans are transmitted to other humans by an arthropodvector, for example, through the bite of a mosquito or sand fly.  Amoebasare members of the sarcodines. They may reproduce both asexually or sexually. Asexual reproduction is a simple celldivision, called fission, in which division of the genetic material can be seenin the nucleus or centre of the cell.

Entamoeba histolyticus is most often spread through theingestion of infected human feces. There are two species of Acanthamoeba thatare free-living: A. castellani and A. culbertsoni. These species can be found infreshwater, 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 viathe blood stream) causes damage to and failure of major organ systems. Symptomsare dependent on the organ system involved but death is the usual outcome.   b) The immunesystem OverviewThe immunesystem is there to keep you healthy by attacking foreign bodies or cellscreated in your body that threaten your health.

When pathogens or infectiousagents such as bacteria, viruses, fungi and parasites attack your body theimmune system begins a series of innate and adaptive defences.Partsof the immune systemSkin – This isone of the most important parts of the immune system because it is the body’sfirst defence against foreign bodies and infectious agents. When the skin isbroken, due to a wound or cracked dry skin, foreign bodies can then entre thebloodstream.Thetonsils and thymus– these organs produce antibodies such as lymphocytes and T lymphocytes.Thelymph nodes and vessels – These make up the lymphatic system which isan important part of the immune system as the lymph nodes filter lymph fluid asit flows through them which traps foreign bodies, which are then destroyed bylymphocytes. The network of lymph nodes and vessels throughout the body carrylymph fluid (nutrients) and waste material between the bloodstream and bodytissues.Thespleen– this organ filters the blood by removing old or damaged blood cells andplatelets and helps the animal’s immune system by destroying foreign bodies.Bonemarrow– Soft tissue found mostly inside long bones of the arms and legs, vertebraeand pelvic bones.

Bone marrow is made up of red marrow, which produces red andwhite blood cells and platelets (important for coagulation), and yellow marrowwhich contains stored fat and connective tissue and produces some white bloodcells.Whiteblood cells– These blood cells are made in the bone marrow and protect the body againstinfection. If an infection develops, white blood cells attack and destroy theforeign body causing the infection.

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

Typesof immune systemsInnateimmunity– 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, andmast cells.

Adaptiveimmunity– This is a specific, acquired immune system that involves highly specialisedcells and processes that eliminate or prevent the growth of pathogens. Adaptiveimmune systems are ‘learnt’ by the body. Once it has fought off a pathogenusing a specialised system the body remembers the process and cells needed, andthen when the same or a similar pathogen attacks the body knows how to react.Naturalimmunity -Naturally acquiredactive immunity occurswhen a person is exposed to a live pathogen, and develops a primary immune response, which leads toimmunological memory. This type of immunity isnatural because it is notinduced by deliberate exposure.Artificialimmunity -Artificially acquiredactive immunity can beinduced by a vaccine, a substance that contains antigen.

A vaccine stimulates aprimary response against the antigen without causing symptoms of the disease.Positivesand negatives of immune responses.The immunesystem is able to create strong and efficient immune responses againstpathogens and parasites without damaging organs in the body. This is mainly dueto the contraction and destruction of the immune response after the infectiousagent has been controlled. During contraction the majority of T cells aredestroyed but the ones left over survive as memory cells to fight a similar orthe same pathogen that infects the animal. However a negative of the generalimmune system is that pathogens are still able to affect our body multiple times,like the common cold.

Morespecifically innate immunity is effective because it means we have something todefend the body from birth. Without this most offspring would die within days. Althoughinnate immunity is still non-specific and in some cases won’t do anything tohelp the body fight the particular pathogen.On the otherhand adaptive immunity is specific, meaning it can efficiently and effectivelytarget and terminate pathogens, however these responses must be learnt, whichmeans the animal would have to have already been exposed to the pathogen. Thiscould be by vaccination, which is close to harmless (unless you have anallergic reaction), or by previously being attacked by the pathogen which couldcause irreparable damage to the animals body c)Bacterial infectionWhen foreignbacteria enter the body they are at first undetected until a stable populationis reached. At this point they change their behaviour and start damaging thebody by altering the environment around them. This causes the immune system tobecome aware of the invasion and activate the macrophages. Themacrophages ingest and destroy the bacteria cells by phagocytosis and also helpto control inflammation by ordering blood vessels to release water in theinfected area so fighting of the infection in that area is easier.

When themacrophages have destroyed all they can they release message proteins that willcommunicate urgency in that location. Upon activation, neutrophils leave theirusual patrol route in the blood and move to the site of infection, where theycreate barriers that trap and kill bacteria. However during the process ofkilling bacteria, they also attack and kill healthy host cells. As they are sostrong they have evolves to commit suicide after 5 days so that they do notcause the host too much damage.

If theinfection hasn’t yet been neutralised, dendritic cells get activated. Thesecells collect dead bacteria, rip them to pieces, and present the parts on theirouter layer. The dendrite cells then travel to the closest lymph node, whichwill take around a day, where multiple helper and killer T cells are waiting tobe activated. The dendrites then look for T cells with a specialised set up tocombine with the shreds of the bacteria cells that are presented on thedendrites membrane. As soon as one is found a chemical reaction takes placewhich activates the T cell. This causes the T cell to rapidly duplicate. Someof these cells remain in the lymph nodes as memory cells, allowing for relativeimmunity if the bacteria attacks again. Others will travel to the site ofinfection to help fight off the bacteria, whilst some travel to the centre ofthe lymph nodes to activate the B cells which then duplicate rapidly andproduces many antibodies to also help fight the infection.

B cells produce somany antibodies that they are at a high risk of denaturing, therefore a helper Tcell stimulates the B cell to prevent it from dying out from exhaustion whilstthe infection is still active. Once the infection is neutralised the B cellswill die to prevent the body from wasting unnecessary energy. Theantibodies produced by the B cells are small proteins designed to bind to thesurface of the intruder. During an infection, millions of antibodies flood theblood and saturate the body until they reach the site of infection, where theydisable bacteria which render them useless or kill them in the process. Theyalso stun the bacteria, making them an easy target for macrophages to ingestand destroy. Once theinfection is neutralised all of the host cells that were destroyed are rapidlyreplaced and most immune cells that are no longer needed destroy themselves.This is effective because it means energy and resources are not wasted onthings like B and helper T cells.

However the bacteria still goes undetectedwhen it first enters the body, until it has reproduced to the point of damagingthe body’s cells. If the bacteria were detected sooner it wouldn’t be able toreproduce enough to cause any significant damage.  ViraldiseasesMacrophagesdestroy germs as soon as they detect them. However, if a viral infection beginsto take hold we fight back using a stronger defence of white blood cells, T andB lymphocytes. Special proteins made by the B cells bind to the virus to stopthem replicating, and also tag them so that macrophages know to destroy them byphagocytosis. For T cells, some guard the body and raise the alarm when theydetect invading viruses; others kill virus-infected cells directly, or help Bcells to produce antibodies. Once the virus has been neutralised, a smallamount 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 isonly using two types of bodily cells that work together efficiently and get towork as soon as a virus is detected.

Also, storing memory cells, once the infectionis over, means that if the virus returns it will be destroyed even faster andmore efficiently than the last. On the other hand, the fact that fighting offthe virus is left down to two types of cells means that if the virus does overpower either the T or B lymphocytes there isn’t many other walls of defenceafterwards. FungalinfectionsWhen thefungi 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 usingpattern recognition receptors on their surface – C-type lectin and toll-like receptorsare the most important in this instance. Once bound to the fungi, the PRR’ssignal using their intracellular tails or associated molecules, which resultsin phagocytosis, the initiation of killing mechanisms such as; the productionof reactive oxygen species, and also create memory cells to help with future,similar infections. This is effective because it initially uses cells from theinnate immune system which means they will come into action almost immediately,instead of relying on the adaptive system to specialise itself to start fightingthe pathogen.

However it is ineffective because in order to actually affect thevirus it needs cells from the adaptive immune system, such as the c-type lectinand toll-like receptors, which will take longer to kick into action.  Parasites- HelminthsResearchshows that parasitic worms have the ability to deactivate certain cells of theimmune system, which means a weakened immune response. This is good for theparasite and the host as it decreases the immune response against harmlessallergens that the worm carries, gut flora, and the hosts own cells.

The immuneresponse is effective in the sense that it’s not causing unnecessary damage tothe hosts body but it is not greatly effecting the parasite itself because theimmune response is stunted.