Infection and response

• Pathogens are microorganisms that cause infectious disease. They include bacteria, viruses protists and fungi.
• Once inside the human body, bacteria reproduce very rapidly, under ideal conditions bacteria can divide every 20 minutes. They can then release harmful chemicals called toxins which damage tissues and make us feel ill.
• Viruses cannot reproduce by themselves, they can only reproduce inside a host cell.

  1. First, the virus invades the host cell.
  2. The virus now reproduces inside the host cell. This is very damaging to the cell.
  3. When the virus leaves the cell, it can cause the cell to burst open and die.
• How pathogens are spread:

  • Some are spread in the air e.g in water droplets.
  • Others can be spread directly in water.
  • Some are spread by direct contact between individuals.
• Reducing the spread of pathogens:

  • Practicing basic hygiene.
  • Providing people with clean drinking water.
  • Reducing direct contact between individuals e.g using a condom.
  • Isolation
  • Vaccination
• Viruses cannot be killed by antibiotics
• The first symptom of measles is often a fever, after around 3 days the patient develops a red skin rash.
• The measles virus is spread in droplets when an infected person coughs or sneezes, the virus then passes into a different person when these droplets are inhaled.
• When infected with Measles, complications may develop. These can cause damage to the breathing system and the brain. In severe cases, measles can be fatal.
• Most children are vaccinated against measles when they are young.
• The first symptom of HIV is often a flu-like illness. The virus is attacking the cells of the patient's immune system. Over time, the immune system becomes severely damaged and later it cannot fight off other infections that others could easily deal with. The immune system is also unable to fight off cancer cells. At this stage the disease is fatal.
• Treating HIV:

  • Antiretroviral drugs stop the virus from multiplying inside the patient so the virus does not damage the patient's immune system
  • Patients who take antiretroviral drugs do not go on to develop AIDS
  • These drugs are not a cure for HIV or AIDS, the patient must take these drugs for the rest of their life.
• Spreading HIV:

  • HIV is transmitted through the exchange of fluids between humans. It can be spread by unprotected sex or when drug users share infected needles.
• Salmonella food poisoning and gonorrhoea are communicable diseases.
• The bacteria that cause salmonella food poisoning are spread by ingesting infected food, food that is prepared in unhygienic conditions. The salmonella bacteria secrete toxins which cause the symptoms of salmonella.
• The symptoms of salmonella are : fever, abdominal cramps, vomitting and diarrhoea.
• Salmonella bacteria are sometimes found in poultry such as chicken, in the UK, all chickens are vaccinated against salmonella. This controls the spread of the disease.
• Gonorrhoea is a sexually transmitted disease (STD/STI). The symptoms include a thick yellow/green discharge from the penis or vagina and pain when urinating.
• Gonorrhoea was easily treated using penicillin but antibiotic resistant strains of bacteria are now common.
• To stop the spread of gonorrhoea we can:

  • use a condom during sex which stops the bacteria passing from person to person.
  • People who have unprotected sex should be tested for gonorrhoea, they can then be treated with antibiotics to kill the bacteria before they pass it onto another person.
• Malaria is a communicable disease. The malaria pathogen is an example of a protist.
• People with malaria experience repeated bouts of fever and it can be fatal
• When a person infected with malaria is bitten by a mosquito, the malaria pathogen passes onto the mosquito. It now bites a different person and passes the malaria pathogen to them. The mosquito is a vector because it carries the pathogen.
• Stopping the spread of malaria:

  • Stop the vector from breeding. Mosquitoes breed in still water, to stop this, we need to find areas of still water and drain them. We can also spray areas of still water with insecticide.
  • Prevent the mosquitoes from biting humans, sleep under a mosquito net, we can also spray the mosquito net with insecticide.
• The job of the non-specific defence system is to prevent pathogens from entering the human body.
• The skin forms a protective layer covering the body. The outer layer of the skin consists of dead cells and is difficult for pathogens to penetrate. It also produces an oily substance called sebum, which can kill bacteria. When the skin is damaged, pathogens are able to enter the body, to stop this, the skin scabs over.
• The nose contains hairs and mucus that can trap pathogens before they enter the breathing system. If pathogens enter the nose and begin moving toward our lungs, the trachea and bronchi are covered with tiny hairs called cilia that are covered in mucus that can trap pathogens. The cilia waft the mucus upwards towards the throat where it is swallowed into the stomach.
• The stomach contains hydrochloric acid which kills pathogens before they can make their way further down the digestive system.
• The immune system destroys pathogens and any toxins they produce, it also protects us in case the same type of pathogen invades us again in the future.
• Phagocytosis

  1. Phagocyte detects chemicals from pathogen

  2. Phagocyte moves towards pathogen

  3. Phagocyte ingests pathogen

  4. Phagocyte uses enzymes to destroy pathogen
• Phagocytes

  Can ingest and destroy pathogens
• Antibodies

  • Protein molecules produced by lymphocytes
  • Stick to pathogens and trigger their destruction
  • Extremely specific and only protect against one pathogen
  • Can remain in blood for a long time to provide future protection
• Lymphocytes


  • Produce antitoxins
  • Antitoxins stick to toxin molecules and prevent them from damaging cells
• Tobacco mosaic virus (TMV) is a widespread plant infection that affects many different plant species such as tomatoes. It causes the leaves to discolour in a mosaic pattern, and because of this discolouration, the rate of photosynthesis is reduced and therefore the growth of the plant is also reduced.
• Rose black spot is caused by a fungus. It causes the leaves to develop purple or black spots, the leaves then often turn yellow and fall off. It causes the rate of photosynthesis to fall and this reduces the rate of growth. The fungus can be spread by water or wind.
• Treating rose black spot:

  • Spray the plants with fungicides.
  • Remove the infected leaves and destroy them.
• Vaccination

  Introducing small quantities of dead or inactive forms of pathogen into the body
• The dead or inactive pathogen in the vaccination cannot lead to the disease in the patient
• How vaccines work


  1. White blood cells are stimulated to produce antibodies against the dead or inactive pathogen

  2. White blood cells divide by mitosis to produce lots of copies of themselves

  3. Copies of white blood cells can stay in the blood for decades

  4. If the same pathogen enters the body later, the white blood cells can produce the correct antibodies quickly to prevent infection
• Vaccination


  Directly protects us from infection by pathogens