B1a4 Infectious Diseases

Attention: open in a new window. PDFPrintE-mail

Article Index
B1a4 Infectious Diseases
Natural Defences
All Pages


Microorganisms that cause infectious diseases are called pathogens.  Examples of some pathogens are shown in the picture below.


Bacteria and viruses can reproduce rapidly inside the body and may produce toxins (poisons) which make us feel ill.  The bacteria which cause food poisoning (e.g. Salmonella and Campylobacter) often produce toxins.  Viruses live inside cells and hijack the cells machinery to make lots of copies of themselves.  They often damage cells when the reproduce and the cell may burst open to release lots of copies of the virus which can then go on to infect new cells.

The human body provides an ideal environment for pathogens to grow and reproduce as it provides a warm environment with a good supply of nutrients.  Under ideal conditions bacteria can reproduce every 20 minutes.

Medicines such as paracetamol and aspirin help to relieve the symptoms of infectious diseases but they do not kill the pathogens.


 Natural Defences against diseases.

The body has different ways of protecting itself agains pathogens.  White blood cells help to defend us against pathogens which may have entered the body.  They protect us in the following ways:

  • by ingesting ('eating')  the pathogens
  • by producing antibodies which destroy particular bacteria or viruses
  • by producing antitoxins which act against the toxins produced by pathogens.


People can be immunised against a disease by introducing small quantities of dead or inactive forms of the pathogen into the body.  This is the process of vaccination.  The vaccine stimulates the white blood cells to produce antibodies that bind to and destroy the pathogen.  The person becomes immune to future infections of the microorganism as the body can rapidly make the correct antibody (as it has been previously exposed to an inactive form of the disease).  The MMR vaccine is used to protect children against three diseases caused by viruses:  measles, mumps and rubella.  This vaccine had been incorrectly linked to an increased risk of autism, a claim that has now been proved to be unfounded.  How vaccines work is shown in the diagrams below:

Girls are now routinely vaccinated against the human papilloma virus (HPV) which can cause cervical cancer.  This virus is spread by sexual contact.  Vaccinating girls before they become sexually active means they are immune to the disease caused by the virus before they can become exposed to it.  The risk of contracting HPV increases with the number of sexual partners a girl has.  If a girl has few (or no) sexual partners there is a very low risk of contracting the disease and the vaccination is not likely to be of benefit.  However, mass vaccination against smallpox and polio has led to smallpox being eradicated from the world and polio is now found in a few countries.  These diseases used to kill or maim (polio causes limbs to become deformed) thousands of people each year.




The first antibiotic, penicillin, was discovered by a scientist called Alexander Fleming.  They are medicines that help to cure bacterial (and fungal) diseases by killing infective bacteria inside the body.  Antiobiotics cannot be used to kill viral pathogens which live and reproduce inside cells.  It is difficult to develop drugs which kill viruses without damaging the body's tissues.

In the experiment shown above, an agar plate is spread with a bacterial culture (staphylococcus aureus in this case).  Discs containing antibiotics are then added to the plate.  In the clear areas the bacteria have been unable to grow - they have been killed by the antibiotic diffusing from the disc.  In the bottom right-hand corner, the bacteria can grow around the disc.  They are resistant to this particular antibiotic.

Antibiotic resistance

Antibiotics have revolutionised our treatment of many diseases.  During the First World War many soldiers died from infections that entered wounds.  The Second World War saw the first widespread use of antibiotics and as a result many injured soldiers survived as infections could be treated.  In modern times we have come to depend on antibiotics to treat many bacterial diseases.  However, the overuse of antibiotics has led to the development of resistant strains of bacteria which are no longer killed by a particular antibiotic.  Pharmaceutical companies are trying to produce new antibiotics but recent research has not led to the development of any new antibiotics.  Now resistant strains of bacteria are causing the deaths of many people in hospitals and MRSA (a bacterium resistant to most antibiotics) is becoming more common.

Bacteria (including MRSA) develop resistance to antibiotics as a result of natural selection.  A particular bacterium may mutate spontaneously to produce a resistant strain  The resistant strain can grow and reproduce in the presence of the antibiotic leading to a rise in the population of the resistant bacteria.  In order to reduce the spread of resistant strains of bacteria, antibiotics are not prescribed to treat non-serious infections (e.g. mild sore throats).  Similarly, viruses can mutate to produce new strains.  For example new strains of the flu virus (such as 'swine flu') arise by mutation.  It can spread rapidly through the human population as few people are resistant to the new strain.  If a disease causes mass infections in one country it is called an epidemic.  Mass infections in different countries are called a pandemic.


Ignaz Semmelweis


Ignaz Semmelweis was a Hungarian Physician who is credited with controlling the spread of infections in hospitals.  He recognised the importance of hand-washing in preventing the spread of some infectious diseases.  By insisting that doctors washed their hands before examining patients he greatly reduced the number of deaths from infectious diseases in hospitals.  The importance of Semmelweis' work is recognised by campaigns run by the NHS in the UK encouraging people to 'Scrub Up'