Microorganisms also known as microbes are minute organisms that cannot be seen with the naked eye. They can only be seen with the help of microscope. Microorganisms are classified into 5 groups: a) Bacteria. b) Viruses. c) Protozoa. d) Algae. e) Fungi.

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Certain microbes such as fungi and bacteria are beneficial while others such as viruses are harmful. Microorganisms are grouped based on the following criteria: a) Size. b) Shape. c) Method of reproduction. d) Nutrition. e) Habitat.

Infection of diseases caused by microorganisms can be prevented by: a) the control of vectors. b) Sterilization. c) Immunization.

Control of Vectors

 A vector is an organism that transmits pathogens from one person to another. Vector control is very effective in controlling the spread of diseases such as malaria and cholera as it is used to reduce the number of vectors by killing or preventing the reproduction of vectors. The life cycles and habits of vectors are two important aspects in the control of vectors. Two main examples of vectors are mosquitoes and houseflies.

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a)  Control of mosquitoes

 

The life cycle of a mosquito shows a complete metamorphosis that goes through that goes through four stages: egg, larva, pupa and the adult (imago). Female mosquitoes lay eggs on the water surface of stagnant water found in drains, ponds or in containers such as empty tins and tires.

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After 2-3 days, the eggs hatch into larvae which hang from the surface of the water. Larvae feed on microorganisms in the water. Larvae change into pupae after 14 days. Pupae do not feed. Pupae undergo metamorphosis and change into adult mosquitoes. Mosquitoes can be controlled at each stage of their life cycle.

b)   Control of Houseflies

Houseflies cause diseases such as cholera and food poisoning. Houseflies live in dirty and smelly places containing decaying organic substances and pathogens like bacteria. The adult housefly lays eggs on decaying organic substances or on the faces of animals and humans. Pathogens from decaying substances attach themselves to hairs on the legs and body of the fly.

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When the housefly lands on uncovered human food to feed, the pathogens fall from its body and legs onto the food. The housefly undergoes a complete metamorphosis. When the eggs hatch, the larvae feed on decaying organic substances. A larva molts twice in 4 days and changes into a pupa. The pupa is a passive stage and is protected by puparium. The pupa then metamorphoses inside the puparium into an adult fly.

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After 6 days, the puparium breaks and the adult fly emerge and flies away. Effective ways of controlling houseflies are as follows: a) eradicating grounds – by burying or burning waste substances and ensuring that rubbish bins are always closed, using modern toilets and cleaning the surroundings of the house. b) Covering food left on tables to prevent houseflies from resting on the food and contaminating it. c) Using insect traps like sticky fly papers to trap flies or using suitable insecticides such as cresol and borax. 

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Sterilization

Sterilization is one method that kills all types of microorganisms and their spores. In this process, pathogens are eliminated or removed from objects or the environment. This method includes the use of heat, chemicals and radiation.

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The methods of sterilization are as follows: a) use of heat- boiling for 6 hours in boiling water at a temperature of 100 degrees Celsius can kill microbes and their spores. This is used to sterilize milk bottles and surgical instruments. – Using and autoclave. Heating surgical instruments for 15 minutes in an autoclave at a temperature of 121 degrees Celsius and high pressure can kill microbes and their spores. b) Use of chemicals-antiseptics is chemical substances which can be applied on the surface of the skin to prevent infections. Antiseptics can also kill pathogens on objects and surfaces such as floors, walls and medical equipment. Examples are Phenol, formalin, hypochlorite, chlorine and sulphur dioxide gases. c) Use of radiation – Gamma rays are radioactive rays that can kill microbes and their spores. They are used to kill pathogens in the contents of closed containers or wrappers. Ultraviolet rays from sunlight can kill microbes. Operating theatres and tissue culture rooms are sterilized using ultraviolet rays from special lamps.

Immunization

Immunization is a process which increases the ability of the body to fight against diseases. Immunity is the ability of the human body to resist infection by pathogens by producing antibodies. When foreign bodies called antigens such as pathogens or vaccines (containing weakened pathogens) enter the body, white blood cells (lymphocytes) are stimulated to produce antibodies (a type of protein) to fight the pathogens. This response is known as immune response.

Antibodies are specific in their actions. One type of antibody fights only one type of pathogen. Immunity can be classified as active immunity or passive immunity. Each type of immunity can be further divided into natural or acquired (artificial). Active immunity refers to the immunity obtained when the body produces its own antibodies. There are two types of active immunity, namely natural active immunity and artificial active immunity.

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Natural active immunity: Immunity is achieved after suffering from disease. When a person is infected with the same pathogen for the second time, the chances of him getting the disease is slim because the body can produce the same antibody in a short time. Sometimes the antibodies produced give life-long immunity from a particular disease. Examples of natural active immunity are immunity to measles, smallpox and yellow fever.

Artificial active immunity: This type of immunity is obtained from vaccinations. A vaccine is a suspension containing dead or weakened pathogens (viruses or bacteria). Examples of vaccines are the Baccilus Calmetic Guerine (B.C.G) vaccine for tuberculosis, the Salk vaccine for poliomyelitis and the vaccines for hepatitis A and B. A vaccine can be given to a healthy person through an injection or droplets.

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When a specific vaccine is given, it stimulates the lymphocytes to produce antibodies to fight against a specific disease. Normally, more than one vaccine is required before a person can achieve an effective level of immunity. For example, to prevent hepatitis B, a person needs three vaccinations. The second and third vaccinations are called booster doses.

Passive immunity refers to the immunity that is achieved when the body cannot produce its own antibodies but receives antibodies or antiserum through outside sources. There are also two types of passive immunity; namely natural passive immunity and artificial passive immunity.

Natural passive immunity: It is achieved when antibodies from the mother is transferred to the foetus during pregnancy or when the baby is feeding on the mother’s milk. This type of immunity does not last long.

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Artificial passive immunity: It is obtained when a sick person is inoculated with antiserum in order to fight a particular disease immediately. It is not permanent and does not last long. Antiserum contains antibodies for a specific disease and serum. Serum is blood plasma from which proteins have been removed.

Antiserum is prepared from animals which are infected by a disease and subsequently recover from it. For example, the antiserum for diphtheria is obtained from horses. A horse is injected with toxin from the Diphtheria bacillus. The horse responds by producing an antitoxin against diphtheria. Serum containing the anti-toxin is prepared from the horse and is then injected into children to fight diphtheria.

Antiserum can also be used to fight diseases such as rabies and tetanus.

Passive acquired immunity: The first injection containing antibodies is given immediately to fight the pathogens. This causes the antibody level to drop slowly below the immunity level. Then a second injection is given to raise the level above the immunity level. If the patient does not recover, subsequent injections are given.

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The Importance of Immunity

 It helps humans from being infected by a particular disease. The human immune system also helps to detect and kill abnormal cells that may cause cancer. The artificial passive immunity is given to patients to prevent the disease from getting worse and to help patients recover faster.

Vaccinations help to prevent the spread of diseases and to prevent an outbreak of a particular disease. Babies and children are protected from various dangerous diseases from an early stage of their lives through immunization programmed.

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The New Immunization programmed in Malaysia

Ages	Vaccines	Types of Disease Prevented
At birth	BCG	Tuberculosis
	Hepatitis B – first dose	Hepatitis B
1 month	Hepatitis B – first dose	Hepatitis B
2 months	DPT + hib – first dose	Diphtheria, tetanus, meningitis,
		whooping cough
	Polio – first dose	Poliomyelitis
3 months	DPT + hib – second dose	Diphtheria, tetanus, meningitis,
		whooping cough
	Polio – second dose	Poliomyelitis
5 months	DPT + hib – third dose	Diphtheria, tetanus, meningitis,
		whooping cough
	Polio – third dose Hepatitis B – third dose	Poliomyelitis Hepatitis B
1 year	MMR – first dose	Mumps, measles, rubella
1 1/2years	DPT – additional dose	Diphtheria, tetanus
7 years	DPT – additional dose	Diphtheria, tetanus
	Polio – additional dose MMR – additional dose	Poliomyelitis Mumps, measles, rubella
15 years	Tetanus –additional dose	Tetanus