B1.1.2 Infectious Disease

B 1.1.2 Infectious Disease


Our bodies provide an excellent environment for many microbes which can make us ill once they are inside us. Our bodies need to stop most microbes getting in and deal with any microbes which do get in. Vaccination can be used to prevent infection.



  • Microorganisms that cause infectious disease are called pathogens.
  • Disease occurs when large numbers of pathogenic micro-organisms enter the body.



  • Not all bacteria are pathogens.
  • Pathogenic bacteria reproduce rapidly inside the body and may produce poisons (toxins) which make us feel ill.
  • Example: coli produces toxins that cause fever symptoms when we have food poisoning.



  • Viruses are much smaller than bacteria.
  • All viruses are pathogens.
  • Viruses also produce toxins and they damage the cells in which they reproduce, leading to illness.
  • Viruses replicate by invading cells, reproducing inside them and bursting them.
  • This causes damage to tissues, leading to illness.

  • Examples:
    • HIV damages white blood cells, reducing immunity and leading to AIDS.
    • Influenza virus released toxins which cause aches and fever symptoms.


The immune system

  • The body has different ways of protecting itself against pathogens.
  • White blood cells defend our internal environment from pathogens
  • These form part of our immune system.
  • There are various types of white blood cells:
    • Cells that ingest and destroy microorganisms

  • Cells that produce antitoxins that destroy toxins released by pathogens
  • Cells that produce antibodies that destroy specific pathogens:
    • They produce specific antibodies to kill a particular pathogen.
    • This leads to immunity from that pathogen.
    • The body is able to rapidly produce large numbers of the specific antibodies if it is exposed to the same pathogen in the future.
    • In some cases, dead or inactivated pathogens stimulate antibody production.


  • This also leads to immunity.

Preventing transmission

  • In the 1850s Semmelweiss recognised the importance of hand-washing in the prevention of spreading some infectious diseases.
  • He insisted that medical students washed their hands before delivering babies.
  • This resulted in doctors washing their hands before and after examining patients.
  • This greatly reduced the number of deaths from infectious diseases in his hospital.
  • This idea was not readily accepted – people were not aware of microorganisms.
  • Nowadays, it is standard practice for people to wash hands after treating patients, to prevent disease being transmitted to other patients.


Using drugs to treat disease

  • Some medicines, including painkillers, help to relieve the symptoms of infectious disease, but do not kill the pathogens.
  • Antibiotics are medicines that help to cure bacterial disease by killing infective bacteria inside the body. Eg penicillin
  • Antibiotics cannot be used to kill viral pathogens, which live and reproduce inside cells.
  • It is difficult to develop drugs which kill viruses without also damaging the body’s tissues.
  • It is important that specific bacteria should be treated by specific antibiotics.
  • Antibiotics kill bacteria inside the body.
  • The use of antibiotics has greatly reduced deaths from infectious bacterial diseases.


Antibiotic resistance

  • Overuse and inappropriate use of antibiotics has increased the rate of development of antibiotic resistant strains of bacteria.
  • Pathogenic bacteria mutate, producing resistant strains.
  • Antibiotics kill individual pathogens of the non-resistant strain.
  • Individual resistant pathogens survive and reproduce, so the population of the resistant strain increases.
  • Antibiotics and vaccinations may no longer be effective against a new resistant strain of the pathogen.
  • The new strain will then spread rapidly because people are not immune to it and there is no effective treatment.
  • Many strains of bacteria, including MRSA, have developed resistance to antibiotics as a result of natural selection.
  • These bacteria can enter the body through wounds and cuts.
  • Healthy people’s white blood cells would quickly destroy these bacteria.
  • People who are ill in hospital are likely to have reduced immunity to bacterial disease, and become infected more easily.


What can be done?

  • Doctor’s should only prescribe antibiotics when necessary – and not for viruses.
  • It is important that if you are prescribed antibiotics you take the whole course.
    • A lot of people will stop taking the antibiotic when they feel better.
    • If you do this, you leave a few bacteria inside your body.
    • These will reproduce, increasing the chance of some developing resistance.
  • Scientists are trying to develop new versions of the antibiotics.
  • Some antibiotics are developed but not used – just in case.


Epidemics and Pandemics

  • Epidemics – diseases that spread widely through one country.
  • Pandemics – diseases that spread through several countries.


Eg Influenza

  • A viral disease.
  • Most people recover in a week.
  • People who are old or very young or already ill can die.
  • Different strains of influenza affect other animals.
  • These rarely affect humans, because humans need to directly contact an infected animal.
  • Humans that are infected may be more likely to die than if they had human influenza.
  • Most of these viruses cannot be transmitted from human to human.
  • However, there are concerns that the viruses could mutate and become able to be transmitted between humans.
  • If it does this, it will start off by causing an epidemic, which may spread to become a pandemic.
  • Many people could die, particularly very old people, very young people, and people who are already ill.



  • If a large proportion of the population is immune to a pathogen, the spread of the pathogen is very much reduced.
  • Eg small pox was completely eradicated by the 1970s.
  • People can be immunised against a disease by introducing small quantities of dead or inactive forms of the pathogen into the body (vaccination).
  • Vaccines stimulate the white blood cells to produce antibodies that destroy the pathogens.
  • This makes the person immune to future infections by the microorganism.
  • The body can respond by rapidly making the correct antibody, in the same way as if the person had previously had the disease.
  • An example is the MMR vaccine used to protect children against measles, mumps and rubella.


Growing Microoganisms

  • Microorganisms = organisms that can only be viewed with a microscope.
  • Eg bacteria, viruses and fungi.
  • Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics.
  • It is important that the culture is not contaminated with other microorganisms that may compete for nutrients or produce toxins.
  • Careful procedures are required to prevent potentially pathogenic microorganisms being released into the environment.


Culturing microorganisms

  • To study microorganisms, they need to be cultured.
  • They need to be provided with the conditions they need to reproduce quickly:
    • Nutrients
    • Warmth
    • Moisture
  • Bacteria and fungi can be grown in special media called agar.
  • This provides them with:
    • Carbohydrate
    • Protein or amino acids
    • Water
  • When agar is heated up it is liquid.
  • It can be poured into a Petri dish.
    • A circular plastic or glass dish with a lid:


  • The agar solidifies when left to cool.
  • Petri dishes and culture media must be sterilised before use to kill unwanted microorganisms
  • Inoculating loops are used to transfer microorganisms to the media.
  • These must be sterilised by passing them through a flame:

  • The lid of the Petri dish should be secured with adhesive tape to prevent microorganisms from the air contaminating the culture.


  • In school and college laboratories, cultures should be incubated at a maximum temperature of 25o
  • This greatly reduces the likelihood of growth of pathogens that might be harmful to humans.
  • In industrial conditions higher temperatures can produce more rapid growth.