Biology - Infection & Response (Topic 3)


Communicable Diseases

Types of Pathogens (Pathogens are microorganisms that cause infectious diseases):   

  • Bacteria: tiny cell 1/100th size of body cells, reproduce rapidly in body, produce toxins that damage body cells and tissues which make you feel ill.
  • Virus : 1/100th size of bacteria cell, not a cell, reproduce inside body cells using their machinery - makes cell burst and release virus, this cell damage makes us feel ill
  • Protist : single celled eukaryotes, some are parasites that live in/on organism and are transferred by vectors.
  • Fungi : some are single celled, others have a body called Hyphae (thread like structure) which can grow and penetrate skin and plant surfaces, they also produce spores which spread the disease  

  How pathogens are spread:                                                                                             

  • Water- through drinking or bathing in contaminated water you become infected. 
  • Air - pathogens can be carried in the air and breathed in, they can also be in water droplets produced when an infected person snezes/coughs and breahing these in infects you. 
  • Direct Contact - by touching contaminated surfaces such as skin or objects that have come into contact with a disease (and not cleaned).
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Viral Diseases(3)

HIV : 

  • Viral disease spread by sexual contact through bodily fluids & sharing needles taking drugs. 
  • Initially causes flu-like symptoms for few weeks then none for years- this early stage can be treated using antiretroviral drugs.        
  • The virus attacks bod's immune cells and if they're badly damaged then it cant cope with other infections or cancers and this is late stage HIV or AIDs.

TMV (Tobaco Mosaic Virus) :

  • virus causes mosaic pattern/dicolouration on leaves
  • This means the plant photosynthesises less and it can't grow as well.


  • Viral disease spread in water droplets
  • symptoms are red skin rash and fever.
  • It can be fatal if there are complicatins such as pneumonia or enchepalitis lung & brain infection. Most people in uk are vaccineated against it.


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Protist & Fungal disease

Rose Black Spot:

  • fungal disase that causes purple/black spots to appear on leaves of rose plants thst can turn yellow and drop off.
  • This causes the plant to photosynthesise less which affects the plants growth.
  • It's spread in the environment through wind and water.
  • Treated using fungicides and striping the plant of its infected leaves then destroying them.


  • Disease caused by a protist
  • Protist is picked up by mosquito feeding on an infected animal and then it infects any other animal it feeds on by inserting the protist into the animals blood vessels. 
  • Symptoms are repeating episodes of fever and it can be fatal. 
  • Prevention of mosquitos breeding reduces spread. 
  • People using insecticides and moquito nets prevents infection.
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Bacterial Diseases

Salmonella : 

  • Bacteria that causes food poisioning 
  • Symptoms are fever, stomach cramps, vomiting and diorrhoea - caused by the toxins bacteia produce.
  • Caught by eating food that was prepaired in unhygeneic conditions or an animal that caught the disease while it was alive. 
  • Most poultry in the uk is vaccinated against it. 


  • Bacteria caught through sexual contact - STD 
  • Causes pain when urineatig and thick yellow/green discharge through penis or vagina.
  • Some strains of the disase are become antibiotic resistant so penicillin is no longer being effective.
  • To prevent the spread treatment is to use antibiotics and barrier method (condoms).
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Reducing/ Preventing spread of Disease

Hygienic - basic hygiene methods - washing hands before preparing food to prevent spread 

Destroying Vectors - vectors are insects that carry diseases - unsing insecticides to kill them and destroying their habitat to prevent them from breding to reduce the spread.

Isolating Infectced Individuals - Isolating infected individuals prevents them passing the disease on to anyone else.

Vaccinations - prevents people and amimals given the vaccine from getting infected and therefore they won't pass the disease on. 

HIV-D :Hygiene, isolation, vaccination, destroy vectors

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Body's defence system

  • Skin is a barrier to pathogens and it secretes antimicrobial substances which kill pathogens.
  • Hairs & Mucua in nose trap particles that could contain pathogens.
  • Trachea and Bronchi lined with cilia which waft the mucus to the back of the throat to be swallowed. 
  • Stomach produces Hydrochloric acid to kill pathogens.

The body's Immune system will kick in to destroy pathogens if they make it in. White blood cells travel in the blood and get into the whole body looking for invading microbes, which there are three lines of attack against 

1. White blood cells will engulf and digest pathogens this is called Phagocytosis.

2. Producing Antibodies - some white blood cells produce proteins called antibodies to lock on to the specific antigens on the invading cells so they can be found and destroyed by other white blood cells. The body then reproduces these rapidly, looking for similar bacteria or viruses in the body.The next time you're infected with this pathogen then your body can rapidly produce antibodies to attack it - Natural Immunity.

3. Producing Antitoxins to counteract the toxins produced by bacteria.

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Vaccination involves injecting a person with a dead/inactive/weakened form of a pathogen so the white blood cells can produce antibodies against their antigens, without the illness.

so if you come into contact with the real thing then your body can rapidly mass-produce antibodies against it. 


  • Controls diseases that used to be common & Eradicated some. 
  • Prevents epidemics- if large amounts of people are vaccinated against it then its less likely to be caught and passed on - Heard immunity. 


  • Don't always work - no immunity.
  • Sometimes bad reactions occur such as fever and seizures.
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Antibiotics & Painkillers

Painkillers: reduce symptoms of the disease but don't treat the pathogens/cause.

Antibiotics: kill bacteria without harming body cells, there are different antibiotics kill different types of bacteria.

Viruses are much harder to develop drugs for as they reproduce inside body cells so it's difficult to kill virus without harming body cells.

Antibiotic resistance: bacteria reproduce by binary fission which means they can mutate to be antibiotic resistant.

  • If an infection has mutated then the non-resistant strains will be killed with antibiotics but the resistant strains will survive and reproduce and increase.
  • This can cause a serious infection like MRSA to become antibiotic resistant. 
  • To prevent this, doctors only prescribe antibiotics for serious infections, if you are prescribed antibiotics it's important to finish your full course.


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Many drugs come from chemicals in Plants

Plants produce chemicals to defend themselves from pests & pathogens. Some we use in drugs to treat diseases or relieve symptoms. Many current drugs come from traditional cures:

  • Asprin used as a painkiller to lower fever, a chemical found in Willow.
  • Digitalis treats heart conditions, a chemical found in Foxgloves.

Some drugs are from microorganisms 

Alexander Flemming noticed bacteria in a petri dish was killed by naturally occurring penicillium mould - Penicillin was the chemical killing the bacteria.

Drugs are now mass-produced in the pharmaceutical industry but many still start from a chemical found in a plant.

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Developing drugs

Preclinical testing:

  • Drugs are tested on human cells and tissues in a lab, this doesn't work if the drug is for drugs that affect whole or multiple body systems.
  • Testing on animals to find the efficacy(how well it works?) the toxicity (does it have any harmful side effects?) and dosage (how much and how often to have?)  - UK law is to test on two live mammals, some say cruel/ some say it's the safest way to test drug.

Clinical tests

  • First, the drug is tested on healthy volunteers to check there are no harmful side effects when the body is working normally, to start off they're given low dosage which is gradually increased. 
  • Next, it's tested on people with the illness to test the optimum dosage
  • Patients are put randomly in two groups one with placebo and one with the drug called blind trials: when the doctor knows who has the placebo but to avoid the results being influenced by the doctor's subconscious a double-blind trial: when the doctor is unaware who has the placebo and who has the drug.
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Monoclonal antibodies (+disadvantages)

Antibodies are produced by B-lymphocytes a type of white blood cell.

Monoclonal antibodies are lots of clones of a single cell, so they're all identical, will only target one specific protein antigen.

Lymphocytes produce antibodies but don't divide easily, however, tumour cells divide rapidly but don't produce antibodies. You can fuse a mouse b-lymphocyte and a tumour cell together to create a Hybridoma cell - which can e cloned to produce lots of antibodies which can be collected and purified

Monoclonal antibodies can be made to bind to anything so this means they will only bind to one specific cell

A main advantage of Monoclonal antibodies is in Cancer Treatment - given that they cause fewer side effects than other cancer treatments (Radiotherapy & Chemotherapy) given that they only target specific cells and so don't harm normal body cells.

However Monoclonal antibodies have more side effects than origionally anticipated: Fever, Vomiting and Low blood pressure, this is unanticipated given that they only target specific cells which was predicted to mean fewver side effects. 

This is why they're not as widely used as origionally anticipated 

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Monoclonal antibodies uses Pregnancy tests

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Plant diseases

Plants need Mineral ions from the soil, a lack of these results in deficiency symptoms.

  • Nitrate is used to make proteins that help the plant grow, deficiency of nitrate results in stunted growth.
  • Magnesium ions are used to make chlorophyll which is used in photosynthesis, lack of magnesium results in a condition called Chlorosis where the plant's leaves turn yellow.

Plants can get diseases from Bacteria, Viruses & Fungus. Signs a plant has a disease are: Stunted Growth, Abnormal Growth, Malformation of stem/leaves, Spots on leaves, Discolouration & Patches of decay.

They can also be infested with pests such as Aphids. If a plant is infested you will be able to see the pests on the plant.

Ways to identify a plant disease:

  • Use gardeners manual/website.
  • Take the plant to a lab where scientists can identify the pathogen.
  • Use testing kits that use Monoclonal Antibodies to identify the pathogen.
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Plant Defences

Physical defences: 

  • Waxy Cuticle on leaves and stem to prevent pathogens entering.
  • Cell wall made from cellulose that surrounds the plant and prevents pathogens getting in if they've already got past the waxy cuticle 
  • Layers of dead cells on the stem as a barrier to prevent pathogen entry - like bark.

Chemical Defences:

  • Antibacterial substances produced by the plant to kill bacteria (Witch Hazel & Mint plant)
  • Poison produced by the plant to deter herbivores from eating it (Deadly nightshade, Foxgloves & Tobacco plant)

Mechanical defences: 

  • Hairs / Thorns on leaves and stem to stop animals touching/eating the plant.
  • Leaves droop/curl to knock off insects and move away from things 
  • Some plants cleverly mimic other organisms (Passion flower has bright yellow spots on its leaves to look like butterfly eggs so butterflies don't lay their eggs there)
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