Biology- Diseases

  • Created by: HannaYB
  • Created on: 02-05-19 19:23

Disease Transmissions

Methods of transmission:

  • Direct Contact (e.g. warts)
  • Water (e.g. cholera)
  • Air (e.g. measles)
  • Vectors (e.g. malaria)

In animals, diseases can be directly transmitted by: bodily fluids, skin-skin contact, faecal-oral route, open wounds, bites, dirty needles and contaminated food. Or, they can be indirectly transmitted via: inanimate objects, droplet infection, flea vectors, or water.

In plants, diseases are usually spread by water, insect or wind vectors. If any part of a diseased plant touching another plant, it can infect it. Plant seed may have fungal spores (mould) and because they can move long distances they can infect other plants. Plants can also become diseased if they come into contact with soil which has had a diseased plant in it.

Vectors - animals which transmit or carry pathogens from one individual to another

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There are four types of pathogens:

  • bacteria
  • fungi
  • viruses
  • protists

Viruses have no nuclei, instead they have genetic material (DNA or RNA). Fungi do have nuclei, as do protists. Bacteria tend to have chromosomal DNA instead of nuclei, and may also have plasmid DNA.

Pathogens - microorganisms which cause disease

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Bacterium vs Virus

How do they multiply?
Bacteria: binary fission (they divide by rapidly splitting in two, outside of cells)
Viruses: replication (they reproduce and live within a host cell)

How do they cause illness?
Bacteria: they produce toxins which affect the body (making you feel ill), and may directly damage cells)
Viruses: they inject DNA into cells within the body, taking them over, before living and reproducing in these host cells (damaging and destroying the host cells)

How can they be treated?
Bacteria: the immune system works against them, and antibiotics can be used
Viruses: the immune system works against them, and antivirals can be used to stop the virus from multiplying (but antibiotics have no effect on them)

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Diseases: gonorrhoea

vaginal dishcarge
painful urination
painful sex


Spread by:
sexual intercourse and contact

Spread prevented by:
using condoms

antibiotics (however, super gonorrhoea is resistant to antibiotics)

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Diseases: HIV

persistent fatigue
nausea and vomiting
rapid weight loss
it eventually leads to the development of AIDs (which stops the immune system from working properly)


Spread by:
sexual intercourse
blood transfusions
sharing needles
from mother to child during pregnancy
(bodily fluids)

Spread prevented by:
using condoms
using clean needles
telling any sexual partners
getting tested
getting medical care during pregnancy


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Diseases: malaria

high fever and profuse sweating

Protist (carried by mosquitoes)

Spread by:
mosquito bites (mosquitoes in areas with higher temperatures may carry the protist and pass it in when sucking a persons blood)

Spread prevented by:
mosquito nets
insect repellent
(there is no vaccination)

antimalarial drugs

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Diseases: measles

sore throat and dry coughs
inflamed eyes (conjunctivitis)
runny nose


Spread by:
droplets from coughs and sneezes (air)

Spread prevented by:
getting the MMR vaccine
getting the Human Normal Immunoglobin (HNIG) treatment (this provides protection for those at immediate risk of catching measles, but is short term)

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Diseases: salmonella

bloody stools and diarrhoea


Spread by:
eating foods contaminated with the faeces of an infected animal
eating undercooked meat or eggs (from contaminated birds)

Spread prevented by:
ensuring meat and eggs are thoroughly cooked
refrigerating foods
not drinking unpasteurised milk
avoiding contact with the bodily fluids of an infected person
cleaning surfaces and washing hands which come into contact with raw meats

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Diseases: rose black spot (plants)

purple or black spotted leaves
yellowed leaves
leaves which fall early
areas of decay or rotting
fewer flowers


Spread by:
wet conditions
contact with other diseased leaves
(water splash)
(fungal spores carried by the wind and water allow the fungus to grow on rose plants, the plant then becomes weakened as the yellowed leaves affect its ability to photosynthesise

Spread prevented by:
avoiding watering foliage (watering roots instead)
allowing air circulation
leaving space between plants

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Diseases: TMV/Tobacco Mosaic Virus (plants)

malformed leaves
distinct yelllowing of veins
mosaic patterned leaves


Spread by:
small wounds caused by handling, insects or broken leaf hairs
(insects can carry the virus to the plant, the virus then destroys the cells in the leaves and affects plant growth as damaged areas of the leaves are unable to photosynthesise)

Spread prevented by:
cutting off infected leaves
soaking seeds in bleach before planting (to kill the virus)

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Diseases: nitrate deficiency (plants)

stunted growth
yellowed leaves
low crop yields

lack of nitrate ions in the soil
(nitrate ions are needed to build up amino acids for building plant proteins)

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Diseases: chlorosis (plants)

yellowed leaves
stunted plant growth

magnesium deficiency
(lack of magnesium in the soil)

(magnesium is needed to make chlorophyll, so without the plant is unable to photosynthesise properly)

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Diseases: aphid infestation (plants)

malformed stems and leaves
weakened plants


(aphids have sharp mouthparts that enable them to feed on phloem sap, they also act as vectors and can transfer pathogens to the plant)

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Defence against disease: non-specific

Hairs and mucus in the nose:
trap pathogens and sweep them out of the body

Skin secretions:
provide a physical barrier, and are antimicrobial

Cilia and mucus in the bronchioles and trachea:
trap pathogens and sweep them out of the body

Stomach acid:
kills bacteria

Platelets in the blood:
help wit blood clotting

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Defence against disease: white blood cells

There are three types of white blood cells, each with a different role:

  • Phagocytosis (non-specific), some white blood cells ingest pathogens, digesting and destroying them to prevent illnesses
  • B-cells (specific), some white blood cells produce antibodies to target particular bacteria or viruses and destroy them
  • T-cells (specific), some white blood cells produce antitoxins which can counteract particular toxins released by pathogens

B-cells make antibodies which are complimentary to different antigens (protein molecules on the surface of pathogens which white blood cells identify as foreign). This is possible because different bacteria or viruses have different antigens, which each have specific/complimentary shapes.

T-cells make antitoxins which target the corresponding toxins (which make us feel ill). Toxins are released by bacteria.

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How vaccines work

  • In vaccines there is a small amount of a dead or inactive pathogen which is injected into the body, these will contain antigens from these pathogens
  • In order to make the pathogen inactive its DNA can be removed, or only its antigen can be injected
  • The injection stimulates lymphocytes (white blood cells) to produce the complimentary antibodies for the pathogen’s antigens
  • This makes the body immune to further infections as the lymphocytes can respond rapidly and make antibodies if the real pathogen enters the body
  • This response will be much faster than without a vaccine

Herd Immunity

  • If a large population is immunised, the spread of a pathogen can be reduced
  • It means the likelihood of coming into contact with someone carrying the pathogen is reduced
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Treating Disease

  • Antiseptics - kill bacteria outside the body
  • Painkillers - relieve symptoms, but do not cure the disease
  • Antivirals - can be used to kill some viruses (e.g. influenza)
  • Antibiotics - kill bacterial cells by rupturing the cell wall

Antibiotics do not harm human cells or viruses as neither animal cells nor viruses have cell walls. In addition, viruses are inside host cells and antibiotics work outside of cells, so viruses are not affected by them anyway. Some bacteria are also resistant to antibiotics.

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Drug Development

Drugs must be:

  • effective (prevents or cures disease, or relieves symptoms)
  • safe (is not too toxic, and doesn’t cause extreme side effects)
  • stable (is storable for long periods of time, and is storable and usable under normal conditions such as temperature)
  • easily delivered (able to reach its target, and be removed once it’s done its job)

Steps for drug development:
1. Lots of new medical drugs are created
2. The drugs are tested and trialled to check safety, efficacy, dosage and toxicity
3. Preclinical testing is carried out on cells, tissues and then live animals
4. Clinical trials are performed on healthy volunteers and patients to check for side effects when low doses of the drug are given
5. The drug is tested on patients to check it treats their disease, if it is safe and effective bigger trials are done to find the optimum dose
6. In double blind trials some patients are given the drug, and others a placebo (which could contain nothing, or a different drug)
7. The results of the trials are published so other scientists can peer review and check the results

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Monoclonal antibodies

How monoclonal antibodies are made:
1. A mouse is injected with an antigen
2. The mouse produces lymphocytes (B-cells) specific to the antigen
3. These are mixed with fast-dividing cancer cells (myeloma)
4. The result is hybridoma cells which can divide rapidly, producing lots of the same lymphocyte
5. All of these cells will make the desired antibody

They can be used for pregnancy test diagnoses, to measure hormone levels in the blood (in labs), to detect pathogens, to locate/identify specific molecules in cells or tissues, to take toxic drugs to cancers (to stop them growing, but only specifically target them).

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Monoclonal antibodies: advantages and disadvantage

they only bind to diseased or damaged cells needing treatment
they can be used to treat a wide range of conditions
they could become cheaper to develop
their side effects are treatable

they are expensive to develop
they’re foreign in the body, so trigger immune responses as side effects
producing the correct antibodies and attaching them to drugs is difficult
it could be considered unethical to source the lymphocytes from mice

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Plant defences

Defence against pathogens:
cellulose cell walls (physical), strengthens plant cells and helps resist invasion by microorganisms, aphids can breach the barrier and give pathogens an entrance into cells
waxy cuticles on leaves (physical), acts as a barrier to the entry of pathogens, the pathogens can still enter at the stomata
layers of dead cells around stems (physical), forms a protective layer hard to breach, when dead leaves are lost or shed pathogens fall off with them
antibacterial chemicals (chemical), protect against invading pathogens, are very effective at preventing bacterial disease in many plants

Defence against herbivores and insects:
poisons (chemical), deter herbivores, animals will learn to avoid plants which make them feel unwell
thorns or hairs (physical), deter insects and larger animals from feeding or laying eggs on leaves or stems, makes it unpleasant or painful for large herbivores to eat
drooping or curling (mechanical), leaves collapse suddenly when touched, dislodges insects and frightens larger animals, is rare but effective
mimicry (physical), plants mimic unhealthy plants or butterfly eggs, stops animals from eating them and butterflies from laying eggs on them

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Immunisation Key Words

Communicable disease: an infectious disease transmissible from one person to a Thor by direct or indirect means

Phagocyte: a cell which engulfs and consumes foreign material and debris

Lymphocyte: a small white blood cell responsible for immune responses such as producing antibodies and antitoxins

Immunisation: the creation of immunity against a particular disease

Natural active immunity: immunity which occurs when a person is exposed to a live pathogen, and becomes immune as a result of the primary immune response from developing the disease

Artificial active immunity: immunity induced by vaccination which involves the production of antibodies

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Cigarettes contain:

  • nicotine (a poisonous chemical which is physically addictive)
  • tar (contains 60+ carcinogens and 4000 different chemicals)
  • carbon monoxide (a poisonous gas which reduces oxygen carrying capacity)

Nicotine affects the heart, blood vessels and brain and increases cholesterol levels, gut movement and blood pressure. Blood circulation is reduced and it increases adrenalin.
Tar collects in the bronchi and bronchioles, paralysing the ciliates cells which remove germs. The germs then cause infections such as bronchitis. It breaks down the alveoli and reduces the surface area for oxygen absorption (emphysema). It contains carcinogens and causes lung cancer.
Carbon monoxide combines with haemoglobin in red blood cells and reduces their oxygen carrying capacity up to 15%. It also increases the amount of fatty deposits in arteries which can cause blocking in the coronary artery (leading to a heart attack).

During pregnancy, carbon monoxide from smoking reduces the mother’s oxygen carrying capacity. The mother also supplies oxygen to her baby, so the fetus may not get enough oxygen to grow properly. This can lead to premature births, low birthweight babies and stillbirths.

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People can become easily addicted and need alcohol to function. They may drink heavily for many years, causing long term damage to the liver and brain (which can kill them). They may develop cirrhosis of the liver (which destroys liver tissue and stops the liver carrying out normal functions). It is also a carcinogen so it increases the risk of developing liver cancer which spreads rapidly and is hard to treat. If the dose of alcohol is too high, it can lead to unconsciousness, coma or even death.

During pregnancy, any alcohol the mother drinks passes through the placenta to the developing baby, which cannot break down or process alcohol. This can damage its organs. Miscarriages, stillbirths, premature births and low birthweights are linked to alcohol consumption during pregnancy. It can cause birth defects and Fetal Alcohol Syndrome (FAS).

Symptoms of FAS:

  • heart, kidney, and bone structure problems
  • learning disabilities and low IQ levels
  • troubles with memory and coordination
  • problems with attention and hyperactivity
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