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Human biology
Biology
Human biology
PSHE / Citizenship studies
Science (applied)
Communicable diseasecommunicable disease
Diseases that can be transmitted from one individual to another.
s cause millions of deaths globally every year. They are caused by pathogens and can be passed from one individual to another. A wide variety of organisms cause communicable diseases, ranging from tiny, geometric viruses to parasitic worms that can grow many metres long inside our bodies. The symptoms of disease result from how of our bodies respond to the invasion of these foreign organisms. Here is a summary of some of the features of common pathogens – to find out more see Pathogens and the immune systemimmune system
The body's natural defence mechanism against infectious diseases.
.
| Type of pathogen | Main features | Examples of disease |
|
Bacteriabacteria
|
Some bacteria invade and destroy the cells. Many produce toxins that cause disease. Endotoxinsendotoxins Lipopolysaccharides that form the outer layer of the cell wall of Gram negative bacteria and act as local toxins to the eukaryotic host cells. act locally, affecting the cells directly around the bacterium. Exotoxinsexotoxins Toxins secreted by bacteria that can cause damage to the host by destroying cells or altering normal cell activity. are released into the body of the host as the bacteria grow and reproduce, travelling around the body causing disease. |
Tonsillitis, septicaemiasepticaemia Severe and often fatal blood poisoning. It is caused when an infection spreads to the blood. , pneumoniapneumonia Swelling (inflammation) of the tissue in the lungs, often initially caused by a virus and often complicated by a secondary bacterial infection. and tuberculosistuberculosis A communicable disease caused by bacteria. It affects millions of people around the world and can be cured by antibiotics, but increasingly the pathogenic bacteria are becoming resistant to the most widely used antibiotics. (TB) |
|
Virus
|
Viruses are a combination of genetic material and protein that can invade living cells. They take over the biochemistry of the cells they invade to make more viruses, often destroying the host cells. | Common cold, influenzainfluenza A viral infection of the breathing system which attacks the lungs and can be fatal. , chickenpox, polio, HIV/AIDSHIV/AIDS The acquired immunodeficiency syndrome (AIDS) is a condition caused by the infection by the human immune deficiency virus (HIV). It attacks and destroys the immune system, leaving the patient defenceless and unprotected from any pathogens and threats. |
|
Fungi
|
Fungi do not cause many human diseases. There are a number of fungal skin conditions, and a few very serious conditions when a fungus attacks internal organs such as the brain or the lungs. | Athlete’s foot, ringworm, aspergillosisaspergillosis A serious type of fungal infection that can affect the lungs, the brain and other organs of people who are already ill or have a compromised immune system. |
|
Protozoaprotozoa
|
Protozoa act as parasiteparasite Organism that feeds off another living host and causes it some damage. An example of a parasite is a tapeworm that lives in the digestive system of a host organism. s and cause human disease. They have two stages in their lifecycle so spend time in humans and another host that acts as a vector. |
Malaria, amoebic dysentery, sleeping sickness |
To cure communicable diseases, medicines must either:
There are a number of different medicines that we can use against pathogens – but viruses and protozoaprotozoa
Protozoa are one-celled animals
are very difficult to destroy.
Bacteria are prokaryotes and there are many features of prokaryoticprokaryotic
A unicellular organism that lacks a membrane bound nucleus or any other membrane bound organelle.
cells that are different from our eukaryoticeukaryotic
Cells that make up animals, plants, fungi and protista. They are three-dimensional, membrane-bound sacs containing cytoplasm, a nucleus and a range of membrane-bound organelles.
cells. Antibiotics are drugs that target bacteria and cure bacterial diseases. Many of their actions affect only bacterial cells, they either kill the bacteria or stop them growing but they do not destroy our human cells.
One way of classifying bacteria is by the way they take up Gram staining – they are known as Gram positive and Gram negative bacteria. The way they take up stain is affected by the structure of their cell walls. The structure of the cell walls also affects how sensitive a bacterium is to different antibiotics – there are some antibiotics that only affect Gram positive bacteria, and others that only affect the Gram negative species. The animation below demonstrates how the different antibiotics affect a bacterium.
The structural differences between the cells walls of Gram positive and Gram negative bacteria affect the way they take up stain
– and the way they are affected by antibiotics
Some antibioticantibiotic
Medicine that is used to treat bacterial infections and works by killing or stopping the growth and reproduction of bacteria. These can be specific to a particular bacteria or act on groups of related bacteria.
s are bacteriocidal – they kill the bacteriabacteria
Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.
. Others are bacteriostatic – they prevent the bacteria from growing and this enables the immune systemimmune system
The body's natural defence mechanism against infectious diseases.
to recognise and destroy the pathogens. It will often depend mainly on the dosage used whether an antibiotic is bacteriostatic or bacteriocidal.
Some antibiotics affect a wide range of different bacteria. They are known as broad spectrum antibiotics. Others only destroy one or two species of bacteria – they are called narrow-spectrum antibioticsnarrow-spectrum antibiotics
Antibiotics that only affect a specific family of pathogenic bacteria.
.
When antibiotics were first discovered and developed, people thought that infectious diseases caused by bacteriabacteria
Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.
would become a thing of the past. There are a number of reasons why this hasn’t happened including:
Antibiotics are only effective if the target microorganism has a binding site for the drug or a metabolic process that the antibiotic interferes with. Mutationmutation
A change in the arrangement or amount of genetic material in a cell.
s that arise during bacterial reproduction may result in phenotypic features that make the bacteria resistant to the drug. For example by making the cell wall impermeable to the drug, changing a biochemical pathway or the production of an antibiotic-destroying enzyme. These mutations will be selected for when the antibiotic is used as they enable the bacteria with the mutation to survive and reproduce when others around them are dying.
Antibiotic resistance has become a major problem – many common antibiotics are now almost useless in many developed countries. Some bacteria have developed resistance to many different antibiotics. If someone is infected by one of these multi-resistant bacteria, it can be a death sentence.
The animation below depicts how antibiotics work, how resistance to them arises and highlights the importance of completing a prescribed antibiotics course. Bacteria are not drawn to scale, i.e., in reality they are much smaller in relation to the size of the petri dish.
What is more, the numbers of new antibioticantibiotic
Medicine that is used to treat bacterial infections and works by killing or stopping the growth and reproduction of bacteria. These can be specific to a particular bacteria or act on groups of related bacteria.
s being developed has been falling steadily. This is partly because it is becoming harder to find new compounds that are effective against bacteriabacteria
Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.
, and partly because the potential financial return is lower because new antibiotics will be used very sparingly to try and avoid future resistance developing.
Graph showing the reduction in the numbers of antibiotics produced over time.
Controlling the use of antibiotics reduces opportunities for pathogens to develop resistance.
e-bug (www.e-bug.eu) is a free educational resource produced by Public Health England for classroom and home use and makes learning about micro-organisms, the spread, prevention and treatment of infection fun and accessible for all students.
