Skoool Kerala

Micro-organisms such as certain bacteria and viruses can cause disease.

Below is a diagram of a bacterial cell. Bacterial cells come in different shapes.  Basically they contain cytoplasm surrounded by a membrane and cell wall made of carbohydrate.  They do not have a proper nucleus, just a chromosome containing genes.

Diseases caused by bacteria include whooping cough, T.B. and salmonella.

Bacteria can be killed by antibiotics.

Viruses

Viruses are much smaller than bacteria and cannot be seen under a light microscope.

A virus has a protein coat surrounding a few genes.

Viruses can only reproduce inside another living cell.  They take over the host cell and the viruses produced quickly infect other cells.

Viruses cannot be killed by antibiotics.

Diseases caused by viruses include colds and flu, chicken pox, measles and AIDS. 

 

Defence Against Disease

The body has several methods of preventing the entry of micro-organisms:

• The skin acts as a barrier
• Mucus produced by cells lining the trachea trap dirt and germs, and tiny hairs called cilia move the mucus up and out of the lungs
• The blood clots and forms scabs to seal cuts in the skin
• Hydrochloric acid produced in the stomach kills microbes

If microbes do enter the body the white blood cells defend the body against disease in several ways:

• Some white cells engulf and digest micro-organisms
• Others produce antibodies which destroy a particular virus or bacterium
• Others produce antitoxins which counteract the toxins (poisons) released by some microbes

Diseases are more likely to occur if large numbers of organisms enter the body due to contact with infected people, or unhygienic conditions.  Disease can be spread by the air, water, food, contact with an infected person or through others animals. 

The symptoms of a disease are usually caused by the toxins that the micro-organisms produce.

To prevent people becoming infected, they may be vaccinated against a disease.

They are given a weakened, or dead, form of the infecting organism.

The white blood cells respond by producing antibodies.

If the organism enters the body at a later time, the white blood cells recognise it and quickly switch on an attack producing antibodies to destroy it.  The person is said to be immune to the disease.

In the UK, people are routinely immunised against T.B., mumps, measles, chicken pox and tetanus.