- An infectious disease is caused by a microorganism entering and attacking your body
- Microorganisms which cause disease are called pathogens
- Common pathogens are bacteria and viruses
- Bacteria are single celled living organisms that are much smalller than animal and plant cells. It is made up of cytoplasm surrounded by a membrane and a cell wall. Inside the bacterial cell is the genetic material. The genetic material is not contained in a nucleus. Although some bacteria cause disease, many are harmless and useful.
- Viruses are even smaller than bacteria. They usually have regular shapes. A virus is made up of a protein coat surrounding simple genetic material. They reproduce by taking over another living cell.
- Once inside the body, they reproduce rapidly. Bacteria split in two and often produce toxins or directly damage cells.
- Viruses take over the cells, damaging and destroying them.
- Common disease symptoms are a high temperature, headaches and rashes which are caused by the damage and toxins produced by the pathogens & your body's resoponse
Spread of disease
Droplet infection: when you cough, sneeze or talk you expel tiny droplets full of pathogens from your breathing system. Other people breathe this in and pick up the infection
Direct contact: direct contact of the skin
Contaminated food and drink: Eating raw or undercooked food, or drinking water containing sewage can spread disease
Through a break in your skin: pathogens can enter your body through cuts, scratches and needle punctures
Preventing microbes getting into your body (primary defence)
- Your skin & hairs cover your body and acts as a barrier
- If you cut your skin you bleed and platelets in your blood quickly help to form a clot which dries into a scab which forms a seal over the cut
- Mucus covers the lining of your lungs and tubes and traps the pathogens
How white blood cells protect you from disease (secondary defence)
- Engulf and digest pathogens
- Produce antibodies which target and destroy bacteria and viruses. The antibodies are specific to that type of antigen (molecules on the surface of the pathogen) and can be rapidly produced if the person is infected with the same pathogen again
- Produce antitoxins
- Recognised the importance of hand-washing in the prevention of spreading some infectious diseases.
- Insisted that doctors washed their hands before examining patients
- Greatly reduced the number of deaths from infectious diseases in his hospital.
The first antibiotic, penicillin, was discovered by Alexander Fleming in 1928. He noticed that some bacteria he had left in a petri dish had been killed by naturally occurring penicillium mould.
- Help to cure bacterial disease by killing infectious bacteria inside the body.
- Cannot be used to kill viral pathogens, which live and reproduce inside cells.
- it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.
- It is important that specific bacteria should be treated by specific antibiotics.
- The use of antibiotics has greatly reduced deaths from infectious bacterial diseases
- Mutations of pathogens produce new strains.
- 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.
- Overuse and inappropriate use of antibiotics has increased the rate of development of antibiotic resistant strains of bacteria.
- Many strains of bacteria, including MRSA, have developed resistance to antibiotics as a result of natural selection.
- Now, antibiotics are not used to treat non-serious infections, such as mild throat infections, so that the rate of development of resistant strains is slowed down.
- People can be immunised against a disease by introducing small quantities of dead or inactive forms of the pathogen into the body
- 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.
- MMR vaccine is used to protect children against measles, mumps and rubella.