Types of Infections

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There are many different types of infection and micro-organisms are integral to infections. To start with we will look at the different types so we can understand how good infection control practices can help protect against infection.

The types of microorganisms are classified as follows:

  • Bacteria are very small organisms that are about one-thousandth to five-thousandth of a millimetre in diameter. Almost all bacteria are so tiny they can only be seen through a microscope. Bacteria are made up of one cell and they are among the simplest single-celled organisms on Earth and were one of the earliest forms of life.  There are probably more individual bacteria than any other sort of organism on the planet. Most bacteria live in the ground or in water, but many live inside or on the skin of other organisms, including humans. There are about ten times as many bacterial cells as human cells in each of our bodies.   Some bacteria can cause diseases, but others help us in everyday activities like digesting food. Some even work for us in factories, producing cheese and yoghurt. 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 cause many common human infections and are also responsible for a number of rare diseases. Examples of viral illnesses range from the common cold, which can be caused by one of the rhinoviruses, to AIDS, which is caused by HIV. Viruses may contain either DNA or RNA as their genetic material.
  • Pathogenic Fungi can be either moulds or yeasts. For example, a mould which causes infections in humans is one that causes of ringworm and which can also infect nails. A common yeast infection is a thrush. 
  • Protozoa are microscopic organisms, but larger than bacteria. Infections caused by protozoa can be spread through ingestion of cysts, sexual transmission, or through insect vectors. A common infectious disease caused by protozoans is Malaria.
  • 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).