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Infectious diseases are caused by microorganisms such as bacteria, viruses, parasites and fungi that can be spread, directly or indirectly, from one person to another. Some are transmitted through bites from insects while others are caused by ingesting contaminated food or water.

How these diseases spread depends on the specific disease or infectious agent. Some ways in which communicable diseases spread are by:

  1. Physical contact with an infected person, e.g. through touch (staphylococcus), sexual contact (gonorrhoea, HIV), faecal/oral transmission (hepatitis A), or droplet (influenza)
  2. Contact with a contaminated surfaces or objects (Norovirus), food (salmonella, E. coli), blood (HIV, hepatitis B), or water (botulism)
  3. Insect or animal bites capable of transmitting the disease (Malaria caused by mosquitos and Lyme disease caused by tick bites)
  4. Airborne, such as tuberculosis or measles.


Over the past two centuries there have been tremendous achievements in control of infectious diseases. Simple measures such as improved sanitation and food safety, along with modern advances in antimicrobials and vaccination programmes have dramatically reduced, and irradiated, many communicable diseases in Northern Ireland; However, with the constant emergence of new organisms and diseases it is important to remain vigilant.

Because infectious diseases can have so much impact on the population, their surveillance and control is an important part of protecting the public’s health. Reporting cases of infectious disease is important in the planning and evaluation of disease prevention and control programs and in the detection of common-source outbreaks.

It is likely that everyone will be affected with an infectious disease at some point in their lives. It is important to learn about the causes of infectious diseases and ways to prevent their spread.

This section of the manual will consider some of the most common infectious diseases/organisms and there is a table included which highlights the incubation period, infection control precautions, transmission route, period of infectivity, and duration of isolation precautions for a list of diseases/organisms.