The most common causative agents or microorganisms can be classified into 4 of the categories already discussed, bacteria, viruses, fungi and parasites. Our bodies are inhabited by many microorganisms, mostly bacteria, which collectively are known as the body’s normal flora. Normal flora does not generally cause disease and are permanent occupants of the human body. Given the numbers of bacteria normally present on the skin, it is to be expected that items such as bed linen, bedside furniture, and other objects in a person’s immediate vicinity will become contaminated with their own normal flora.
Pathogenic bacteria are those that have the potential to cause disease. This ability is influenced not only by the properties of the bacteria but also by the ability of the host, human or animal, to resist infection. Some normal flora microorganisms can become opportunistic pathogens. This means that they can cause infection if tissue invasion occurs at specific body sites i.e. when a person receives a cut on their hand, a type of bacteria known as Staphylococcus aureus, a normal flora microorganism, could enter the body via the cut and potentially develop an infection.
Viruses are smaller than bacteria and require a living cell/host (e.g. human or animal) to grow and multiply. Outside the host cell or human body, the virus particle is known as a virion and it no longer grows or multiplies, however it can remain infectious or viable for a period of time.
In one study, both influenza A and B viruses survived for 24-48 hr on hard, nonporous surfaces such as stainless steel and plastic but survived for less than 8-12 hr on cloth, paper, and tissues.
Norovirus, a common cause of gastrointestinal outbreaks in healthcare settings and cruise ships, characterized by profuse diarrhea, are particularly hardy in the environment. Scientists and medical authorities agree that Human Immunodeficiency Virus (HIV) does not survive well outside the body, making the possibility of environmental transmission remote.
Fungi are larger than bacteria and found in the environment, particularly in dirt, dust and wet environment. Often, fungi are specifically referred to as yeasts or moulds.
Fungi of concern are moulds known as Aspergillus. This organism is most found in the dust in ceiling and wall spaces and can cause very serious disease in immunocompromised persons when disturbed. This occurs mainly during construction and renovation. Other environmental sources of Aspergillus outbreaks in hospitals have been associated with open windows, contamination of particleboard frames surrounding air filters, and backflow of contaminated air from exhaust ducts (with contamination of filters).
Parasites are defined as any organisms living in or on any other living creature and deriving advantage from doing so, while causing disadvantage to the host. They can be internal parasites (e.g. Giardia – an intestinal parasite), blood parasite (e.g. malaria) or an external parasite (e.g. lice, scabies).
A parasite can stay permanently with their host or may spend only a part of their life cycles in association with the host.