Viruses are tiny microorganisms that cannot be classified as either eukaryotic or prokaryotic entities. However, viruses can cause disease in either prokaryotic or eukaryotic organisms. A topic of conversation in the scientific community is whether viruses are living or non-living entities. You may wonder, how could viruses not be alive when they can reproduce and propagate, causing disease in other organisms? Well, viruses only really 'live' once they have invaded a cell. We define 'living' by whether an entity, like a cell, carries out a list of metabolic processes, like respiration, excretion and its ability to replicate itself, among other features. Viruses are strange because they carry out some of these processes independently but notably can't replicate themselves as cells do, the basic units of life. However, once a virus invades a host cell, it hijacks the host cell's machinery and is able to carry out these processes. Viruses are sort of halfway between living and non-living! Keep reading to learn more about viral diseases, viral infections, and more.
Viral Disease Meaning
Viral diseases are some of the most common diseases that affect the human population. Think about your common cold and flu and COVID-19! These are all types of viral diseases, and as you know, these can be spread from person to person because viral diseases are infectious diseases. Sneezing, coughing, and other modes of transmission can spread viral diseases from one person to another. The exact method of transmission depends on the pathogen itself.
They're all kinds of infectious diseases caused not only by viral pathogens but also bacterial pathogens, for example. These microorganisms circulate amongst us and can infect people, animals and plants. Read more about this in our Communicable Diseases article.
Viral Diseases: Viral Infection
As mentioned before, viruses are neither prokaryotic nor eukaryotic cells. They are too small to be seen with most microscopes and are not considered living cells. This is because viruses need a host (e.g. animal or plant cell) to reproduce and stay alive.
Viruses can contain either DNA or RNA. The genetic material is often found within a protein coat. Viruses do not reproduce like other pathogens. Instead, they replicate their genetic material and protein coats within the infected host cell. After replication, they then escape by bursting out of the host cell. After blasting the cell, they find other host cells to invade and repeat the process. Have a look at Figure 1 to see the general structure of a virus.
Viral Communicable Diseases
Viruses can be transmitted in different ways. For example, HIV can be spread via sexual contact and the exchange of bodily fluids. Ebola is another example of a virus spread by contact with bodily fluids. Other viruses such as norovirus are spread via direct contact with an affected person.
We will look at HIV and measles in more detail below, as well as the tobacco mosaic virus.
As we discussed above, coughing and sneezing are two common modes of transmission of respiratory viruses. These methods are described as respiratory transmission. There is a common misconception that with respiratory transmission, the particles released from the sneeze or cough need to land directly on someone else for the viral disease to be transmitted. This is not the case. Instead, the airborne particles released from the sneeze, or cough, can remain in the air for hours. They can also land on other surfaces, like tables or chairs. When someone touches the area, they can become infected with the viral disease. This is why cleaning down surfaces and coughing and sneezing into a tissue, or the inside of our elbow is essential. The COVID-19 pandemic has highlighted this for us all, and hopefully, we will be more careful going forward!
What's the difference between an epidemic and a pandemic? An epidemic is a disease that spreads over a small area. A pandemic is a disease that affects many people over a much larger area, spanning many different countries and continents.
Treatment of Viral Diseases
When discussing the treatment of illnesses caused by different pathogens, we need to understand how the pathogen causes its disease to tailor the treatment strategy to the specificities of the pathogenic microorganism. For example, for bacterial infections, we use antibiotics. For fungal infections, we use antifungal medications. For viruses, we use antiviral medication.
The same way antivirals can't cure a bacterial infection, antibiotics can't cure a viral infection!
Many diseases caused by viruses, such as AIDS caused by HIV, are incurable. This means that the condition will never completely go away. However, the symptoms can be almost completely dealt with by antiviral drugs.
Another way that we can help to defend ourselves against viral diseases is by getting vaccinated. Vaccination involves getting a weakened or modified version of a pathogen and injecting it into our bodies to mount an immune response against it. As our body has prepared the immune response, it makes the secondary immune response much faster and more effective, meaning we are less likely to become sick if we are infected by the whole pathogen. This is the reason behind the COVID-19 vaccinations and the annual flu jab!
Vaccinations are a type of active immunity. This is immunity achieved by our body creating its own white blood cells, specifically lymphocytes. As our body has made its own white blood cells, memory cells remain in the blood allowing the body to mount the same immune response repeatedly whenever your white blood cells come into contact with the same virus. Vaccinations are one of the most impressive achievements in biomedical research! Read more about them in our article Vaccination.
The flu pathogen changes every year because the antigen found on the virus's surface changes constantly. This is due to a phenomenon described as antigenic drift. Small genetic mutations cause slight changes in the antigen's shape, meaning memory cells created after prior vaccinations no longer recognise the new strains of the virus. Due to this, the flu jab needs to be adjusted each year to the new strain of the influenza virus.
Types of Viral Infection
Viral infections are not limited to only affecting humans. They can affect livestock and plants as well. The transmission route for each of these viral infections will, of course, be different depending on the cells involved, and the types of diseases will also change.
Cattle can be affected by Foot-and-Mouth Disease (FMD). This disease can affect both domestic and wild hoofed animals and can be fatal in some cases. FMD is spread by direct contact. FMD can also spread from animals to people, making it even more dangerous as cattle and other livestock are usually found near humans. For humans, FMD typically goes away on its own and does not require treatment. However, if cattle in the UK have FMD, they must be destroyed.
Plant viral infections rarely kill plants. Instead, they often cause discolourations and/or stunt growth. Viruses that affect plants are usually immobile, meaning they cannot move by themselves. Instead, they require vectors that carry the virus from one plant to another. Viral infections can cause huge damage to vegetable crops, and so cause devastating economic impacts.
Vectors are organisms that transport a pathogen from one organism to another without being affected by the pathogen themselves. An example of a vector is the mosquito in the transmission of different plasmodium parasites which cause malaria.
An example of a viral disease that affects plants is the tobacco mosaic virus and the turnip mosaic virus. Turnip mosaic virus causes discolouration, leaf distortion and stunting in plants including cabbage and brussel sprouts. Many viral diseases can not be treated, instead, farmers have to isolate and kill any infected plants to make sure that the virus is not transmitted further to other plants, reducing the economic impact that the virus can have.
Examples of viral diseases
Below we expand on three specific viral infections or diseases of importance, specifically measles, HIV and TMV.
Measles
Measles is a virus that commonly affects younger children. Measles is transmitted through airborne droplets. This means it is spread via sneezes and coughs. Once infected, measles causes a red rash on the skin and also causes fever. Measles is a very contagious disease. In many developing countries, such as countries in Africa and Southeast Asia, many people still die every year from measles infections.
Like the flu and COVID-19, measles is also a virus that affects the respiratory tract. It is also spread via airborne transmission, through coughing and sneezing. Measles infections can either be fairly mild or extremely dangerous. In children, measles infections are more likely to lead to death than adults. However, the number of deaths caused by measles has now reduced significantly due to a worldwide vaccination programme, which has seen almost 90% of children receive at least one measles vaccination.
Smallpox is another example of a viral disease. It's the only infectious disease ever considered entirely eradicated by the WHO! Like the other viral diseases discussed above, smallpox was a disease that affected the respiratory tract. Smallpox was a viral disease that killed millions of people over thousands of years. An extremely successful vaccination programme in the 1960s led to the complete eradication of the virus! It is now only found in secure scientific laboratories and cannot be found naturally occurring anywhere.
HIV
HIV (Human Immunodeficiency Virus) is a virus spread via sexual contact and contact with bodily fluids of an infected person. This can spread by people sharing the same needles. Once infected by HIV, the virus causes a flu-like sickness. This includes fever, cough and general nausea. However, the major problems occur once AIDS develops. AIDS (Acquired Immunodeficiency Deficiency Syndrome) refers to the damage to the immune system caused by HIV. This usually takes a long period to set in after the initial infection. Have a look at Figure 2 to see the structure of HIV.
As we discussed above, AIDS is not curable. However, with the use of antiviral drugs, the symptoms can be treated. HIV infection almost always led to death in the past, but nowadays we have much better antiviral drugs and many people with an HIV infection go on to lead regular lives.
Tobacco Mosaic Virus
Another example of a viral disease is the tobacco mosaic virus (TMV). Unlike the examples we have discussed, the tobacco mosaic virus affects plants rather than animals. Tobacco mosaic virus affects tobacco and other plants, specifically damaging the chloroplasts of the plants.
Remember, chloroplasts are the organelles responsible for photosynthesis. They also contain chlorophyll, which means they are responsible for the plant's colour. Chloroplasts are found in plant cells but not in animal or bacterial cells. This explains why the tobacco mosaic virus affects plants but not other organisms.
When a plant is affected by the tobacco mosaic virus, the leaves get a mosaic pattern and lose colour. Photosynthesis can no longer occur, so the plant cannot grow. As photosynthesis is the process that allows a plant to grow and thrive, without photosynthesis, plant growth is stunted, potentially even leading to plant death. Tobacco mosaic virus infection can have a substantial economic impact on farmers.
Infection by tobacco mosaic virus cannot be cured, only treated on a larger scale. Due to this, farmers have to try to contain the virus within a certain area to prevent it from affecting more crops. This is because the tobacco mosaic virus is transmitted by contact between plants. If the virus is not contained, it can spread like wildfire from one plant to another, wreaking havoc on tobacco crops and potentially losing farmers thousands of pounds. Have a look at figure 3 to see the tobacco mosaic virus.
As we've seen in this explanation, viruses can infect both plants and animals, and even bacteria! Many of the infections lead to disease, with the effects ranging from mild discomfort to death. However, viruses can be treated with antiviral drugs and vaccines that protect the population from even getting infected to start with!
Viral Diseases - Key takeaways
- Viruses are not prokaryotes or eukaryotes, they are non-living entities.
- Viral infections are treated using antiviral medication.
- Measles, HIV and Ebola are examples of viruses affecting animal cells.
- Tobacco mosaic virus is a virus that affects plant cells.
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