Infectious diseases can be caused by a number of different pathogens such as bacteria, parasites and viruses. There has been a lot of discussion recently about a virus that has caused a pandemic, which you can read about in our other blog posts here and here. But what exactly is a virus?
Scientists believe that there are more than 100,000,000 different viruses in existence. This means that there may be a hundred million times more virus particles on Earth than there are stars in the universe. However, although there are a huge number of them, very few cause disease.
What are Viruses?
Viruses are small, much smaller than bacteria. We measure them in nanometres (1 nanometer is 10-9m or 0.000000001m). The largest known virus is 440 nanometers, compared to bacteria which are around 1000 nanometers long. That means you could fit 500 million common cold viruses on the head of a pin. In order to see them, we must use a very fancy piece of equipment called an electron microscope which allows us to see all the various shapes and sizes that viruses come in.
A virus is made up of 2 basic components. The first is the viral genetic material. This can be in the form of either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) which encodes between 3-100 viral genes depending on the virus. We use this genetic code to classify viruses into families. A virus can reproduce itself even if there is nothing but the viral genetic material present in the cell. Therefore, the viral genetic material alone is infectious. The sole ambition of the virus is to get into cells and deliver this genetic information so that more virus particles can be produced. The second component is protein. The viral genetic material is surrounded by a protective layer of protein which stops it getting degraded and also allows the virus particle to bind to its target receptors on the cell surface. This protein layer is called the viral capsid. Some viruses (as in the case of SARS-CoV-2) also have an outer envelope. The virus acquires the envelope from the lining of the host cell. When the virus leaves the cell, it covers itself in part of the cellular lining which helps to disguise it from host’s immune cells.
Viruses do not solely affect humans but can infect all forms of life including other animals, plants, insects and even bacteria. Some viruses can be very host-specific while others can infect a broad range. For example, human immunodeficiency virus (HIV) only infects specific types of human white blood cells while influenza virus can infect many different species of mammals and birds.
Although viruses are known to be able to survive outside of a host for long period of time, they are completely dependent on the host cell in order to reproduce. Once they infect a susceptible host, they hijack the cell’s machinery allowing the virus to replicate thousands of times. However, unlike bacteria they do not reproduce by dividing but instead they replicate their genetic code and protein coats. These can then assemble into new virus particles which are released from the infected cell and spread to non-infected cells.
Virus Transmission
Viruses can be transmitted between individuals in many different ways. Some viruses spread in infected droplets from coughs and sneezes. These droplets can either be breathed in or transferred from surfaces (like door handles) to your hands and then to your nose and mouth when you touch your face. Viruses that affect the respiratory system (like SARS-CoV-2 and influenza viruses) are often transmitted this way. Once in your nose or throat, they very quickly infect the cells that line these tissues and then start to replicate. Other viruses are transmitted by body fluids such as blood or semen (e.g. HIV) or via a bite from an infected animal (e.g. rabies virus from dogs) or insect (e.g. Zika virus from mosquitoes).
Immune Response to Viral Infections
No matter how you get infected, your body has many defences that will kick in to try to protect you from the virus. The immune response is specific for that pathogen and will very effectively destroy virus-infected cells, but it takes a while for this immune response to develop. This is why a cold makes you feel ill for a few days but you then get better once the cells and antibodies of the immune system kick in. Once a targeted immune response has developed it will remember that virus and be able to respond more quickly if it sees it again; this “immunological memory” is why you will rarely get a disease like chickenpox twice – the next time you’re exposed to the chicken pox virus, your immune system remembers how to fight it and destroys it before it can cause disease.
Antivirals and Vaccination
While the immune system is very effective, it’s not always able to control an infection before it causes a lot of damage, or even kills the host. This is where vaccines or anti-viral drugs are very helpful. Anti-viral drugs are designed to treat an existing infection and are usually specific to sub-groups of viruses. For example, anti-viral drugs used to treat HIV have been designed specifically to inhibit the action of HIV genes, and usually don’t work (or don’t work very well) on other viruses.
Unlike anti-viral drugs, which treat an existing infection, vaccines aim to prevent the initial infection. Vaccines usually use individual (non-pathogenic) viral proteins, or an inactivated form of a virus, to trick the immune system into thinking that you’ve been infected with the full virus. The immune response to the vaccine protects you from later exposure to the pathogenic virus. This can be extremely effective and many viral diseases have now become rare as a result of effective vaccination programmes. Smallpox, an extremely dangerous virus that historically killed millions of people, was eradicated in the 1970s after a rigorous global vaccination campaign. Polio is another viral disease that is close to eradication after a huge vaccination campaign, while it is expected that cases of cervical cancer caused by human papillomavirus infection will be hugely reduced as a result of the papillomavirus (HPV) vaccine, first introduced in Scotland in 2008.
There has been much debate over the last two decades over whether vaccines are safe, and many of you will have come across articles in the media suggesting that vaccines cause other diseases, particularly autism spectrum disorders. The scientific evidence on this is clear: huge clinical trials show there is no link between vaccines and autism (a good scientific review of the evidence for this can be found here). No vaccine is completely free of side effects but scientists develop vaccines that are far less likely to cause complications than the diseases that they are designed to protect against. However, the dangers of not vaccinating are clear: areas of the world where vaccination against measles has fallen are showing measles outbreaks (see here and here) and complications of measles infection can be severe, including inflammation of the brain or death. Vaccines save lives.
If you’d like to find out more about viruses, have a look at the Art Goes Viral colouring book produced by the MRC – University of Glasgow Centre for Virus Research.