NOTES

Causes of Spread of Infection

Bacteria
Bacteria are tiny living beings known as micro-organisms. They are usually a few micrometers in length and normally exist together in millions. They come in 3 main shapes: Spherical, Rod Shaped and Spiral and can be found everywhere. There are approximately 10 times as many bacterial cells in the human body then human cells, and a lot of these are found lining the digestive system. Some types of bacteria are useful for production of foods such as yoghurt.

The cell structure is simpler than that of other organisms as there is no nucleus or membrane bound organelles. Instead their control centre containing the genetic information is contained in a single loop of DNA.

Some bacteria have an extra circle of genetic material called a plasmid. The plasmid often contains genes that give the bacterium some advantage over other bacteria. For example it may contain a gene that makes the bacterium resistant to a certain antibiotic.

Bacteria reproduce by binary fission, which is when a single cell divides into two identical daughter cells, each with identical DNA to the parent cell. The daugther cells then divide again and the process continues. When conditions are favourable such as the right temperature and nutrients are available, some bacteria like e.coli can divide every 20 minutes. This means that in 7 hours one bacterium can generate 2,097,152 bacteria. After one more hour the number of bacteria will have risen to a colossal 16,777,216. That’s why we can quickly become ill when pathogenic microbes invade our bodies.

Virus
A virus is a small infectious agent that replicates inside the living cells of other organisms (The host). Viruses can infect all types of life forms, including animals, plants and bacteria. There are millions of different types. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. The shapes of viruses range from simple helical and icosahedral forms to more complex structures. The average virus is about one-hundredth the size of the average bacterium.

A virus is made up of a core of genetic material, either DNA or RNA, surrounded by a protective coat called a capsid which is made up of protein. Sometimes the capsid is surrounded by an additional spikey coat called the envelope. Viruses are capable of latching onto host cells and getting inside them. The virus particles burst out of the host cell into the extracellular space resulting in the death of the host cell. Once the virus has escaped from the host cell it is ready to enter a new cell and multiply.

Parasites
A parasite is an organism that lives in another organism called a host. It depends on its host for survival and often harm's it by taking its nutrients so it can live, grow and multiply. A parasite cannot live independently and although it rarely kills the host, in some cases it can happen. There are several types of parasites.

Fungi
Fungi can be single celled or very complex multicellular organisms. They are found in just about any habitat but most live on the land, mainly in soil or on plant material. A very small number of fungi cause diseases in animals, and in humans these include