Published on Mar 16, 2022
Authored by Dr Cassandra Richmond, Associate Medical Director HBU
Medically reviewed by Pfizer Hospital Medical Team
As you may be aware, there are a whole range of viral infections in existence - and these may cause a variety of different illnesses (think: cold sores, the ‘flu’ and COVID just to name a few, albeit important, ones). But what are viruses, and how do they cause infection? Importantly, what general measures can you take to protect yourself?
Viruses exist as tiny (even smaller than microscopic!1) particles, called virions2. Each virion contains a genetic core2 (consisting of either RNA or DNA), surrounded by a protein shell2, known as a capsid. Some viruses are also enclosed in an outer membrane or envelope2. As one of the most abundant life forms on earth3 (if you believe they are actually alive – but more on that in a moment), it’s fair to say that viruses are almost everywhere!
Outside of a host cell, viruses exist as inactive entities3. However, once they have infected a cell, they utilise the energy from their host to begin replicating3 (and replicating, and replicating, and so forth…).
Because of their inert nature when they are on their own, some experts question whether viruses are, in fact, living organisms3. On the other hand, others believe that viruses are living parasites that have cleverly evolved over time and are pared down to consist of only what is absolutely necessary for their survival3.
Virus particles are well-designed to infiltrate host cells – and they utilise a variety of means to invade the body to cause infection4.
The first step for the virus in infecting a host (e.g. a human being) is for it to find an access point. For many viruses (such as rhinoviruses, coronaviruses, and influenza viruses), the most common route for entry is through the respiratory tract (mouth and nose)5. Microbes such as enteroviruses often invade through the gastrointestinal tract6 – for example, when a person consumes contaminated foods or water. Human immunodeficiency virus (HIV), hepatitis, and herpes viruses often gain entry through the urogenital area during sexual activity7. Viruses can also enter through the eye, as well as through skin where there has been a breach in its integrity (such as from a skin abrasion)5. Thankfully, the body contains very effective natural barriers to protect against many viral infections, including the production of tears, stomach acid, the flushing out effect of urine flow, and the presence of intact skin8.
Once virus particles make it inside the host’s body, the next step is for them to bind to host cell surface receptors2 and inject their genetic material inside the host cell1. At this point, the virus begins to reproduce1. If the host does not deactivate these processes, infection occurs. Depending on the site of infection, and the type /strain of virus involved, the person may start to develop symptoms. You know these well – fevers and chills, headache, cough, runny nose, nausea and vomiting, diarrhoea… the list goes on.
The replicating virus may also spread to other cells and tissues in the body, often by entering the bloodstream5. This may cause a localised infection to become more generalised5 – and the person may become even sicker.
Thankfully, the immune system jumps into action as soon as it recognises a viral invasion. Did you know that developing a fever is actually an important protective response by the body to fight infection9? By raising our core temperature, we are in effect being quite unhospitable to the virus – as this overheats the virus particles and destroys them. Further, in its defence, the body releases non-specific antibodies and other proteins to bind to the viral particles to block them10. It also produces virus-fighting immune cells (such as Natural Killer cells10) – and you can imagine what might happen here! Once the body has fought off the infection, the immune system often develops immunity. This means it ‘remembers’ a previous infection, and mounts a quicker, greater response to the same virus if/when it becomes re-exposed.
If we have previously been vaccinated against a particular virus, our body is better equipped to fight that virus when it encounters it in real life. In a nutshell, when a person comes across a virus that they have been vaccinated against, the immune system generally ‘remembers’ how it responded to viral-like proteins present in the vaccine, and mounts an immune response to prevent the actual virus from taking over and causing an infection11.
The pandemic has highlighted the importance of implementing public health measures to prevent or minimise viral infections generally.
To prevent viral transmission, it is important to:
Last reviewed 31/03/2022
TAPS DA 2230DG
-Australian Government: Flu (influenza)
-Australian Government: Rotavirus
-Australian Government: Coronavirus (COVID-19) pandemic
-Health Navigator NZ: Viral infections | Pokenga huaketo