Topic 3 - Infection and Response

Infection and Response GCSE AQA revision cards :)


communicable disease

Pathogens are microorganisms that enter the body and cause disease. They cause communicable diseases that spread easily.

Types of pathogens:

  • Bacteria - they can make you feel ill by producing toxins that damage your cells and tissues.
  • Viruses - they live inside your cells and replicate themselves using the cell's machinery to produce many copies. The cell get so full of viruses they burst, this releases all the new viruses. This cell damage is what makes us ill.
  • Protists (eukaryotes) - some protists are paracites, paracites live on or inside other organisms and can cause them damage. They are often transferred to the organism by a vector which doesn't get the disease itself.
  • Fungi (single celled) - they have a body which is made up of hyphae (thread-like structures). These hyphae can grow and penetrate human skin and the surface of plants, causing diseases. The hyphae can then produce spores, which can spread to other plants and animals. 
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how do pathogens spread?

  • Water - pathogens can be picked up by drinking or bathing in dirty water.
  • Air - pathogens can be carried in the air can then can be breathed in.
  • Direct contact - pathogens can be picked up by touching a contaminated surface, including skin.
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viral diseases

  • Measles - spread by droplets from an inflected person's cough or sneeze. Symptoms: red skin rash; fever. Most people are vaccinated against it when they are young.
  • HIV - spread by sexual contact or exchanging bodily fluids. Symptoms: flu-like symptoms for a few weeks; usually a person does not experience any symptoms for a few years. HIV can be controlled by antiretroviral drugs - stops virus replicating. The virus attacks the immune system - if the immune system caannot cope with other infections or diseases, this is known as AIDs.
  • Tobacco Mosaic Virus (TMV) - affects plants. It causes a mosaic pattern on the leaves of a plant (discolouration). The discolouration means photosynthesis cannot be carried out well, so it affects the plant's growth.
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fungal diseases

  • Rose black spot - causes purple/ black spots to develop on the leaves of rose plants, the leaves can turn yellow and drop off. This means that less photosynthesis can happen, so the plant doesn't grow very well. It spread through wind or water. It can be treated using fungicides and by ********* the plant of its affected leaves - the leaves need to be destroyed so that the fungus can't spread to other plants.
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protist diseases

  • Malaria - part of the malarial protist's life cycle takes place inside a mosquito. The mosquitoes are vectors which pick up the malarial protist whilst feeding on the infected animal. Every time the mosquito feeds on another animal, it infects it by inserting the protist into the animals blood vessels. Symptoms: fever; can be fatal. Prevention: insectisides and mosquito nets.
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bacterial diseases

  • Salmonella - causes food poisoning. Symptoms: fever; stomach cramps; vomiting and diarrhoea. The symptoms are cause by toxins that the bacteria produce. You can get salmonella by eating food that is contaminated.
  • Gonorrhoea - sexually transmitted disease (STD). Passed on by sexual contact. Symptoms: pain when urinating; thick yellow/ green discharge. It used to be treated with penicillin (antibiotics) but the gonorrhoea strain had become resistant. Prevention: use protected sex; and people with it should be treated with antibiotics.
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preventing disease

  • Be hygienic - using simple hygiene methods can prevent the spread of disease.
  • Destroying vectors - by getting rid of the organsims that spread the disease, you can prevent the disease being passed on. Vectors that are insects can be killed using insectisides or by destroying their habitat so that they can no longer breed.
  • Isolating infected individuals - if you isolate someone who has a communicable disease, it prevents it from being passed on to anyone else.
  • Vaccination - vaccinating people and animals against communicable diseases means they can't develop the infection and then pass it on to someone else.
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fighting disease - body defenses

Body's features to stop pathogens entering:

  • Skin - acts as a barrier and it secretes antimicrobial substances to kill pathogens.
  • Hair and mucus (nose) - trap particles that could conain pathogens.
  • Trachea and bronchi - secrete mucus to trap pathogens.
  • Cilliated cells - these are hair-like structures, which waft mucus up to the back of the through where it can be swallowed.
  • Stomach - produces hydrochloric acid to kill pathogens that make it that far from the mouth.
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fighting disease - immune system

If pathogen make it into your body, the immune system can destroy them. White blood cells travel around the blood looking for microbes. When they come across an invading microbe they have three ways of killing it.

  • Phagocytosis - engulfing foreign cells and digesting them.
  • Producing antibodies (B-lymphocytes) - every invading pathogen has antigens on its surface. When some types of WBC come across a foreign antigen they will start to produce proetins called antibodies to lock onto invading cells so that they can be found and destroyed by other WBC. The antibodies produced are specific to that type of antigen. Antibodies are then produced rapidly and carried around the body to find similar viruses or bacteria. If a person is infected with the same type of pathogen again, they will not become ill as they are naturally immune as the body already knows what antibodies to create (stored in the memory cells).
  • Producing antitoxins - these counteract the toxins produced by invading bacteria.
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fighting disease - drugs

Painkillers are drugs that relieve pain without killing the disease or pathogen causing the underlying problem - they help reduce the symptoms.

Antibiotics kill/ prevent the growth of the bacteria causing the problem, without killing any body cells. Different antibiotics kill different types of bacteria. Antibiotics dont destroy viruses because they reproduce using your own body cells, this makes it difficult to create a drug that destroys only the virus and not the body cell.

Origins of drugs:

Plants produce chemicals to defend themselves against pathogens and pests, some of these chemicals can be used as drugs to treat or relieve the symptoms of human diseases. Some drugs were extracted from microorganisms.

  • Asprin - used as a painkiller to lower a fever. It was developed from a chemical found in willow.
  • Digitalis - used to treat heart conditions. It was developed from a chemical found in foxgloves.
  • Penicillin - mould produces a substance (penillin) that kills bacteria.
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resistant bacteria

Bacteria can mutate, this may cause them to be resistant to antibiotics. This means that if you have an infection, some of the bacteria may be resistant to antibiotics - when you treat the infection only the non-resistant bacteria will be killed. The individual reistant bacteria will survive and reproduce, the population of the reistant strain will increase - this is an exmaple of natrural selection.

To slow down the rate of developing resistant strains, it is important for doctors to avoid over-prescribing antibiotics. It is also important to finish a whole course of antibiotics and don't just stop when you feel better.

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developing drugs

Preclinical testing - human cells and tissues in the lab.

Drugs are tested on human tissues in the lab, however you cannot use human cells and tissues to test drugs that affect the whole body or multiple body systems.

Preclinical testing - live animals.

This is to test for whether the drug works and produces the effect you are looking for; to find out its toxicity; and to find out the best dosage it should be given in. Ethics - some people think that animals are so different to humans the procedure is pointless (cruelty).

Clinical trails - human volunteers.

  • First the drug is tested on healthy people - this is to make sure there are no harmful side effects. At the start a low dosage of the drug is given and is gradually increased.
  • Then the drug is tested on people with the illness to find the optimum dose (most effective, fewest side effects).
  • Then, a placebo is used in a double-blind trial to make sure the drug is producing the desired effects without people/ doctors making up a suitable result to the trail - helps prevent false claims.
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monoclonal antibodies

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monoclonal antibodies - uses

  • Pregnancy tests - a hormone called HCG is found in the urine of pregnant women only. Pregnancy testing sticks detect this hormone. The part of the stick you wee on has some antibodies to the hormone, with blue beads attached. The test ***** has more antibodies to the hormone stuck so it so they cannot move. If you are pregnant: the hormone binds to the antibodies on the blue beads. The urine moves up the stick, carrying the hormone and the beads. The beads and hormone bind to the antibodies on the *****. So the blue beads get stuck on the *****, turning it blue.
  • Treating diseases - different cells in the body have different antigens on their cell surface, so you can make monoclonal antibodies that will bind to specific cell in the body. Cancer cells have antigens on their cell membranes (tumour markers). In the lab, you can make monoclonal antibodies that will bind to the tumour markers. An anti-cancer drug can be attached to the monoclonal antibodies to stop the cancer cells growing and dividing. The antibodies target specific cells because they only bind to the tumour markers. The drug kills the cancer cells but not normal body cells.
  • In the lab and in research - monoclonal antibodies can be used to: bind to hormones and other chemicals in blood to measure their levels; test blood samples for certain pathogens; and locate specific molecules on a cell or in a tissue (using fluorescent dyes).
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plant diseases

Plants needs mineral ions from the soil, if there are not enough ions they can suffer dificiency symptoms.

  • Nitrates are needed to make proteins for growth. Lack of nitrates causes stunted growth.
  • Magnesium ions are needed for making chlorophyll, which is needed in photosynthesis. Lack of magnesium ions causes chlorosis and for plants to have yellow leaves.

Plants can be infected by viral, bacterial and fungal pathogens. They can also be infested with insects. Some signs a plants has a disease are:

  • Stunted growth; abnormal growths; spots on leaves; malformed leaves or stems; patches of decay; and discolouration. You can see pests on a plant.

Different diseases have different signs, these can be identified by:

  • Looking them up in a gardening mannual/ website.
  • Taking an infected plant to a lab where scientists can identify the pathogen.
  • Using testing kits to identify the pathogen using monoclonal antibodies.
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plant defences


  • Most plant leaves and stems have a waxy cuticle, this provides a barrier to stop pathogens entering. Plant cells are surrounded by cell walls made from cellulose, these form a physical barrier against pathogens that make it past the waxy cuticle. Plant shave layers of dead cells around their stems that stop pathogens entering.


  • Some plants have antibacterial chemical that kill bacteria. Other plants have poisons which can deter herbivores.


  • Some plants have adapted thorns and hairs which stop animals from touching and eating them. Other plants have leaves that droop or curl when something touches them. This prevents them being eaten by insects by knocking them off. Some plants mimic other organisms which tricks animals into not eating them.
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