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Infection - It's causes and spread

The Causes of Infection

Micro-organisms are integral to infections, and a basic insight into the characteristics of commonly encountered micro-organisms is essential for good infection control practice. Micro-organisms that cause disease are referred to as pathogenic organisms. They may be classified as follows:
Bacteria are minute organisms about one-thousandth to five-thousandth of a millimetre in diameter. They are susceptible to a greater or lesser extent to antibiotics.
Viruses are much smaller than bacteria and although they may survive outside the body for a time they can only grow inside cells of the body. Viruses are not susceptible to antibiotics, but there are a few anti-viral drugs available which are active against a limited number of viruses. Viruses can cause very serious disease and are found in the blood and body fluids of infected persons.
Pathogenic Fungi can be either moulds or yeasts. For example, a mould which causes infections in humans is Trichophtyon rubrum which is one cause of ringworm and which can also infect nails. A common yeast infection is thrush caused by Candida albicans.
Protozoa are microscopic organisms, but larger than bacteria. Free-living and non-pathogenic protozoa include amoebae and paramecium. Examples of medical importance include Giardia lamblia, which causes an enteritis (symptoms of diarrhoea).
Worms are not always microscopic in size but pathogenic worms do cause infection and some can spread from person to person. Examples include threadworm and tapeworm.
Prions are infectious protein particles. For example the prion causing (New) Variant Creutzfeldt-Jakob Disease (vCJD).

The Spread of Infection

There are various means by which micro-organisms can be transferred from their reservoir to susceptible individuals. These are:
Direct Contact. Direct spread of infection occurs when one person infects the next by direct person-to-person contact (e.g. chickenpox, tuberculosis, sexually transmitted infections etc.).

Indirect Contact. Indirect spread of infection is said to occur when an intermediate carrier is involved in the spread of pathogens e.g. fomite or vector.
A fomite is defined as an object which becomes contaminated with infected organisms and which subsequently transmits those organisms to another person. Examples of potential fomites are instruments, trays or practically any inanimate object.
Crawling and flying insects are obvious examples of vectors and need to be controlled.

Hands.
The hands of practitioners are probably the most important vehicles of cross-infection. The hands of patients can also carry microbes to other body sites, equipment and staff.

Inhalation.
Inhalation spread occurs when pathogens exhaled or discharged into the atmosphere by an infected person are inhaled by and infect another person. The common cold and influenza are often cited as examples, but it is likely that hands and fomites (inanimate objects) are also important in the spread of respiratory viruses.

Ingestion.
Infection can occur when organisms capable of infecting the gastro-intestinal tract are ingested. When these organisms are excreted faecally by an infected person, faecal-oral spread is said to occur. Organisms may be carried on fomites, hands or in food and drink e.g. Hepatitis A, Salmonella, Campylobacter.

Inoculation.
Inoculation infection can occur following a “sharps” injury when blood contaminated with, for example, Hepatitis B virus is directly inoculated into the blood stream of the victim, thereby causing an infection. Bites from humans can also spread infection by the inoculation mode.

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