Year 9 Infectious Diseases
Name: ______________________________
Information on Microbes & Infectious Diseases
What are microorganisms?
Microorganisms (sometimes called microbes) are the smallest living things. They are so small that they cannot be seen
without the help of a microscope. There are four types of microorganisms you need to know about:
Bacteria, Viruses, Fungi and Parasites
Many microbes are useful to human, animals and plants. Those who are harmful to other living organisms are known as
pathogens, germs or pathogenic microbes. Infectious diseases are caused by pathogenic microorganisms, such as
bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one
person to another.
Bacteria
Bacteria are simple organisms that contain only one cell (they are single-celled). Their cell is
different to animal and plant cells because it contains no nucleus. It is also much smaller than
an animal or plant cell. Bacteria can be harmful, in which case they are often known as germs.
Harmful bacteria can cause food poisoning or very serious diseases like tuberculosis (TB),
tetanus or meningitis. However, many bacteria are harmless to humans; human skin is covered in bacteria and the
human digestive system contains many bacteria that cause no problems at all. Bacteria need a food supply to allow
them to grow and multiply. Many bacteria are involved in the decomposition
Viruses
Viruses do not have any of the structures found in normal cells. They do not need food. They are
made of a protein coat that contains a strand of DNA. They need to enter a cell of another living
thing to reproduce. They then use the cell to make new copies of the virus to spread to other
organisms. Viruses are even smaller than bacteria. Diseases caused by viruses include the
common cold, flu, chicken pox, measles and AIDS.
It is a point of argument whether viruses are actually alive, because they don’t grow, respire, excrete etc.
Fungi
Not all fungi are microorganisms; mushrooms, for example, are fungi that can be seen without a microscope. Fungi that
are microorganisms include yeast, penicillin and the organism that causes Athlete’s Foot.
Parasites
These include protozoans, parasitic worms, flukes, fleas, mites etc.
The uses of microorganisms
It is a mistake to think that all microorganisms are harmful; often, humans use microorganisms for more useful
purposes:
Yeast (a fungus) respires when sugars are present to form two substances; carbon dioxide and alcohol. This process is
called fermentation. We can use the carbon dioxide when baking bread to make the dough rise, so yeast is used in
bread-making. We can use the alcohol to make alcoholic drinks, so yeast is used in the manufacture of beer, wines or
spirits.
Penicillin (another fungus) makes a form of antibiotic (see below) for fighting against bacteria. Many microbes can be
used to make vaccines including bacteria and viruses.
Both bacteria and fungi are known as decomposers in nature – breaking down dead bodies and wastes to useful
materials for plant uses.
How do harmful microorganisms spread?
(Mode of Transmission of Infectious Diseases)
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person-to person - by blood (e.g. by sharing needles),saliva (e.g. by kissing) or, for very
infectious diseases, through the air (e.g. by coughing or sneezing)
by food: bacteria may survive in food that is not cooked properly or that is reheated and
give you food poisoning that makes you vomit.
by water: contaminated water may spread diseases like typhoid or cholera.
by insects: many insects are spread by insects. The plague (Black Death) for example,
was carried by fleas living on rats.
How can we protect ourselves against infectious disease?
(A) Protect the pathogens (germs) from entering the body:
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Your body has many natural defences against harmful microorganisms entering and causing harm:
The skin acts as a barrier against microorganisms entering.
Hairs in your nose and in the passageways leading down to the lungs are coated in mucus that traps
invaders. The hairs can then push the mucus up and out when you cough, sneeze or blow your nose.
Enzymes in tears and saliva break down bacteria to stop them entering your body.
If you eat or drink something that contains bacteria, they are often destroyed by the acid in your stomach.
(B) Combat the pathogens once they get into the body:
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Your blood contains white blood cells and platelets (as well as red blood cells for carrying oxygen):
White blood cells – 2 main groups:
(i) Phagoctyes can ingest (eat) bacteria in your blood or from between your body’s cells.
(ii) Other white blood cells are known as “lymphocytes” that can produce special chemicals called antibodies
that destroy microorganisms.
If an area of the body becomes infected, many white blood cells will
rush to that area to attack the microorganisms that have invaded.
Pus is the remains of thousands of white blood cells that have died
defending your body from infection!
Platelets help your blood to clot if your skin is cut. This stops
bacteria from invading using the open wound and, of course, stops
the bleeding.
Antibiotics
Antibiotics are drugs that doctors prescribe against bacterial infections. They do not work against diseases caused by
viruses, which is why doctors will not give them to patients with a common cold or flu. The most famous antibiotic is
called penicillin. It was discovered by a Scottish scientist called Alexander Fleming.
Immunisation (Vaccination)
Immunisation is the process of injecting people with dead or inactive versions of microorganisms (vaccination). Once
a person has been vaccinated, their body starts to make antibodies that destroy the microorganism. Because the
microorganisms in the vaccination are not dangerous, the person does not get ill. However, if they later come into
contact with a live (dangerous) microorganism, then they already have the antibodies they need to destroy it before it
multiplies and makes them ill.
TASK: You are required to make a Mind Map using an A3 Paper (with your partner). You must include the
following big headings
Combat germs
after entry
Treatment
Prevent germs
from entry
Prevention
What is it?
MICROBES
Transmission
of diseases
Infectious
Diseases
4 Types
Useful
Harmful