The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [85]
The life cycle of all viruses is similar and can be summarized as follows (Goering et al. 2007).
1. Attachment: a virus particle attaches to the outside of a host cell. Viruses are generally very limited in the types of cell that they can attach to, and normally infect only a single species or a limited range of related host organisms. Even a wide-ranging virus such as rabies is restricted to infecting mammals.
2. Penetration: the virus particle enters the host cell. The exact mechanism of this depends on the virus and the type of host.
3. Uncoating: the virus particle breaks down and exposes the viral genetic material.
4. Replication: the instructions contained in the viral genes cause the host cell mechanisms to create more viral particles.
5. Release: the new viral particles are released from the cell. Some viruses may ‘bud’ from the surface of the cell, acquiring their enclosing membranes in the process, but often release occurs due to cell rupture and destruction.
This process is illustrated in Figure 3.3.
Figure 3.3 The viral life cycle. Used with permission from Perry (2007).
A final point to consider in relation to viral structure and infection prevention and control is the presence or absence of a lipid envelope enclosing the viral particle. Those viruses that have a lipid envelope, such as herpes zoster virus (responsible for chicken pox and shingles), are much more susceptible to destruction by alcohol than those without, for example norovirus, which is a common cause of viral gastroenteritis.
Fungi
Like bacteria, fungi exist in many environments on Earth, including, in some cases, as commensal organisms on human beings. Unlike bacteria, they are eukaryotic, so their cells share some characteristics with other eukaryotes such as humans, but they are distinct from both animals and plants. Fungi are familiar to us as mushrooms and toadstools and the yeast that is used in brewing and baking. They also have many uses in the pharmaceutical industry, particularly in the production of antibiotics. Fungi produce spores, both for survival in adverse conditions, as bacteria do, and to provide a mechanism for dispersal in the same way as plants (Goering et al. 2007).
A few varieties of fungi are able to cause opportunistic infections in humans. These are usually found in one of two forms: either as single-celled yeast-like forms that reproduce in a similar fashion to bacteria, by dividing or budding, or as plant-like filaments called hyphae. A mass of hyphae together forms a mycelium. Some fungi may appear in either form, depending on environmental conditions. Fungal infections are referred to as mycoses. Superficial mycoses such as ringworm and thrush usually involve only the skin or mucous membranes and are normally mild, if unpleasant, but deeper mycoses involving major organs can be life threatening. These most frequently occur in patients who have severely impaired immune systems and may be an indicator of such impairment; for example, pneumonia caused by Pneumocystis jiroveci (previously carinii) is considered a clinical indication of AIDS. Superficial infections are generally transmitted by physical contact, whereas deeper infections can result, for example, from spores being inhaled. This is why it is important to ensure that patients with impaired immunity are protected from situations where the spores of potentially pathogenic fungi are likely to be released, for example during building work (Goering et al. 2007).
Protozoa
Protozoa are single-celled animals, some species of which are medically important parasites of human beings, particularly in tropical and subtropical parts of the world where diseases such as malaria are a major public health issue. Unlike bacteria, their relationship with humans is almost always parasitic – that is, their presence has an adverse effect