Introducing our new series “All About…” where we delve into various topics. Where better to start, then the thing that’s on everyone’s minds and on everyone’s tongues – viruses.
What is a virus?
Viruses are infectious particles, 100 times smaller than bacteria, which can only be observed using electron microscopes. Individual virus particles are known as virions and are about 20 – 250 nanometres in diameter.
Structure of Viruses (This is about to get sciency)
Viruses are acellular structure meaning that they do not have cellular structure; lacking cellular components such as organelles, ribosomes and the plasma membrane. Virions have an outer envelope made of protein and phospholipid membrane derived from the host cell. A virion consists of a nucleic acid core of either DNA or RNA creating the viral genome. Viruses may also contain additional proteins, such as enzymes.
Viruses are generally categorised based on the type of DNA or RNA they contain, that is double-stranded or single stranded; positive or negative. The Baltimore classification of viruses, is the most common, and has 7 classification groups based on this method. Viruses can be further distinguished by whether they are segmented; where the genome (the DNA or RNA) is separated into different segments, or non-segmented; where the genome is in one segment. Viruses may also have their genome in a circular structure.
Every virus has a capsid, which is an outer protein structure that forms a protective shell to protect the viral genome. Some viruses only contain capsids, and are known as non-enveloped and those that contain a capsid as well as an outer lipid membrane and are known as enveloped viruses.
|Non-Enveloped Viruses||Enveloped Viruses|
|Capsid Only||Has a capsid and also contains an outer lipid membrane|
|More resistant to environmental factors (temperature, pH, disinfectants)||More susceptible to environmental factors|
|Can survive in water||Can be inactivated by 70% alcohol|
|Many viruses that cause intestinal infection are non-enveloped||Respiratory illnesses|
Bacteriophage is a special type of virus that infects bacteria, and have a head, collar and tail, and appear to look like an insect.
Transmission of Viruses
Viruses can be transmitted in various ways including through a vector, such as a mosquito; via direct contact, as is the case in Ebola; blood-born, such as Hepatitis C; animal bites, as in the case of rabies and through droplets that enter the respiratory system, such as Severe Acute Respiratory Syndrome (SARS).
Viruses come in many shapes and sizes, but these are consistent and distinct for each viral family. Even though viruses may be in the same family, they may have different transmission mode and disease progression.
Where do new viruses come from?
Viruses have been around for a very long time, and each virus infects certain species. We have been living with many of these viruses for a long time. The real panic occurs when a virus jumps a species, so to speak. This is called zoonotic infections. We are constantly exposed to potential new viral pathogens from other animals, especially when we come in close contact with them, particularly when they are stressed. However, successful emergences are rare, and include influenza, HIV, SARS and Ebola.
Why do you get sick?
Viruses can be seen as intracellular parasites as they must attach to a living cell, be taken inside, manufacture its proteins and copy its genome, and find a way to escape the cell so that the virus can infect other cells. Viruses can infect only certain species of hosts and only certain cells within that host. Viruses make you sick by disrupting normal cellular function and killing cells. The immune system responds often with fevers and other symptoms to combat the virus and eliminate it, and this is why this a common symptom in most viral infections.
Treating the Virus
A common myth is that antibiotics can be used to treat viruses, but this is not the case. Antibiotics can only treat bacteria, as they disrupt bacterial cellular process. However, as viruses depend on the host’s cellular processes, and do not have their own cellular processes, antibiotics can’t be used to treat viral infections, like they are used for bacterial infections.
Instead, anti-viral drugs are used to treat viral infections; however, they are very limited in their effectiveness. These drugs have had limited success in curing viral disease, but they have been used to reduce symptoms. Most anti-viral drugs work by inhibiting the action of certain proteins. But it is important these proteins are not present in healthy cells of the human, otherwise the drug will also kill normal healthy cells. In this way, viral growth is stopped without damaging the host.
However, the primary method of preventing viral diseases, is vaccination. They are intended to prevent severe viral infections by exposing the body’s immune system to the viruses; and therefore, the immune system builds immunity against the virus. Vaccinations is the process of introducing a weakened virus into the body, so that the body can eliminate the virus with greater efficiency and speed if it were to be reintroduced.
Examples of Viruses
Examples of viruses, include Ebola, measles, mumps, chickenpox, HIV, influenza, and coronaviruses such as SARS, MERS and the common cold.
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