What is a Virus and What Does it Do?

What is a Virus?

A virus is a type of tiny microbe. It consists of genetic material (DNA or RNA) inside a protective protein coating.

A virus needs a host organism (bacterium, plant or animal) to provide it with the processes it needs to thrive and reproduce.

Infection by a virus can result in disease (including colds and flu in humans, and leaf mosaic disease in plants). Occasionally an infection can result in the death of the organism.

What do viruses do?

The virus will invade cells of the host, and once inside, it will use that cell to replicate itself and make more viruses. This process will often damage or destroy the infected cells.

An individual virus will usually infect a specific host species, or group of species. However, cross-species transmission can sometimes occur and the virus will then be able to infect a new host species. Many viral diseases which have affected humans in recent years, including COVID-19, Avian Flu, SARS, and Ebola, are thought to have passed from animals to humans.
Sometimes a virus will infect a new host species, but will not be able to spread between others of that species. In some cases – such as with Coronavirus (SARS-CoV-2) or Ebola – the virus is infectious and is able to spread from person to person.

A particular virus will usually infect specific type of cells in your body such as those in your liver, respiratory system, blood or skin.

Viruses are extremely tiny and are said to be so small that 500 million rhinoviruses (which cause the common cold) could fit on the head of a pin. Most viruses are too small to be seen with an optical microscope, but they have been photographed with scanning electron microscopes.

There are many different shapes and types of virus. Some viruses (including influenza and coronaviruses) are surrounded by a lipid membrane; these types of viruses are called enveloped viruses.

What is a Virus and what does it do ? - 6 viruses
What is a Virus and what does it do ? – 6 viruses – designed by brgfx / Freepik

Some diseases caused by viruses

Viruses are responsible for numerous diseases in humans, including the common cold, influenza, chickenpox, measles, hepatitis, smallpox, HIV/AIDS and rabies.

A viral disease is any illness or health condition caused by a virus.

Common Cold

There are many different viruses which can cause a cold; the majority of colds are caused by rhinoviruses or coronaviruses.

Cold infections are caused by more than 250 virus serotypes belonging to at least five different families. The most common cold-causing viruses are rhinoviruses (RVs) (10 to 50% of all colds), coronaviruses (10 to 15% of all colds), and influenza viruses (5 to 15% of all colds)
Journal of Virology/American Society for Microbiology (ASM)

More than 200 different viruses are known to cause the symptoms of the common cold. An estimated 30-35% of all adult colds are caused by rhinoviruses. In people with asthma, particularly children, rhinovirus infections are also frequently associated with flare-ups. Scientists had previously identified 99 distinct rhinovirus types. Recently, however, a number of unknown types were detected in patients with severe flu-like illnesses.
Understanding a Common Cold Virus – National Institutes of Health


There are 3 main types of influenza virus which can infect humans.

Type A and B influenza viruses are responsible for the seasonal flue outbreaks.

  • Influenza type A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, seals and cats.
  • Influenza type B viruses circulate widely only among humans.

Type C influenza virus usually causes mild respiratory symptoms.

Type A influenza is the only influenza virus which is known to cause global flu pandemics.

A flu vaccine can help to protect you from types A and B influenza viruses. There is no vaccine available for type C influenza virus.

Influenza virus inside
Influenza virus inside – Source CDC

How do viruses spread?

Viruses spread from one host to another in numerous different ways. Some viral diseases are contagious (they can be spread by person to person contact). Others are transferred by insect bite, animal bite, contaminated water, or other methods.

Some ways that viruses can be spread:

  • Coughs & sneezes & respiratory droplets (cold, flu. coronavirus)
  • Transfer of bodily fluids (e.g. HIV/AIDS, ebola)
  • Insect bite (e.g. West Nile virus, Zika)
  • Animal bite (e.g. Rabies)

The spread of many contagious viruses, such as influenza, the common cold, and coronavirus, can be reduced by improved respiratory hygiene and regular hand washing.

Why do viruses make you ill?

The symptoms you have when you get a cold or flu are due to your body?s attempts to get rid of the virus; they are not directly caused by the virus itself.

Our bodies will attack viruses and any other foreign bodies. The body’s immune response can include an inflammatory response and fever. Numerous chemicals and killer cells are used by the body in an attempt to remove the invading virus (including antibodies which can destroy the virus, and cytotoxic T cells which destroy infected cells). Memory cells can retain a memory of the virus for the next time, and will help create immunity to that virus.

Antibodies and immunity

Antibodies are an important feature of your immune system and they help to fight invaders such as viruses. Antibodies are able to lock on to the virus and neutralise or destroy it.

When you have immunity to a virus (due to vaccination or previous infection with that virus), your body can create antibodies which are able to quickly recognise and attack the virus, therefore preventing or reducing the severity of the disease.

Virus mutations

Many viruses can change (or mutate) over time, and small changes to the structure of the virus can affect its characteristics and its behaviour.

Some viruses (such as influenza viruses) mutate very regularly. The flu virus changes very fast and this is why a new flu vaccine needs to be made available every year. It is not possible to maintain immunity to flu and cold viruses for very long and it is difficult to create vaccines for viruses which regularly mutate.

This is also why the idea of building up “herd immunity” for coronavirus (SARS-CoV-2) COVID-19 may not work. It is likely that any immunity may be short lived (possibly just a few months) and once the virus mutates you can be infected again.

Treatments for viral diseases

Unlike bacterial infections, viral diseases don?t respond to the use of antibiotics. The treatment of viral disease is usually focused on support, rest and hydration.

Vaccines are available for some viruses, and these can either protect you from becoming infected or reduce the severity of the symptoms if you do become infected.
Vaccinations can help to reduce the spread of a virus, by creating “herd immunity” in the population. Some vaccines can last for a lifetime, but some are only effective for a short time.

Where do viruses come from?

It is thought that many of our viruses originate in animals. A virus is sometimes able to cross the species barrier, moving from animal to animal or from animal to human – most likely due to a mutation in the virus which slightly alters its structure and allows it to successfully infect a different host.

The current Coronavirus (SARS-CoV-2) outbreak is thought to have started in a wet market in Wuhan, China. These markets typically sell all sorts of live animals, fish and birds (including wild animals) which are often kept in appalling conditions and with a general lack of hygiene. In these markets many different species of animal are kept in close contact with each other and with humans, giving plenty of opportunity for diseases to spread (and a chance to spread between different species which in nature would probably never come in contact with each other).
There have been many calls to ban these types of markets which are common in China and other parts of Asia.

Scientists studying the current coronavirus outbreak think it is likely that it originated from bats, and then passed to humans via an intermediate animal. This would be similar to the SARS outbreak in 2002: another coronavirus which moved from horseshoe bats to cat-like civets, then infected humans.

Ultimately it is human behaviour which is responsible for these viruses making the jump from animals to humans.

Virus transmission and infectivity

How easily a virus can spread from person to person can vary. Some viruses (such as measles) are highly contagious and spread very easily, while other viruses do not spread as easily.

The new coronavirus that causes COVID-19 seems to be able to spread quite easily.

Scientists use a measure of how infectious a disease is, called the basic reproduction number – also known as R0 or ?R nought?.
This number tells us how many people, on average, each infected person will go on to infect. Therefore it tells us how infectious a disease is.

Influenza has an R0 value of 1.5, so each person with influenza will typically infect 1.5 people (on average).

Here are the basic reproduction numbers for some well known viral diseases:

MERS 0.8
Influenza 1.5
Ebola 2.0
COVID-19 2.5
SARS 3.5
Mumps 4.5
Rubella 6.0
Smallpox 6.0
Measles 16.0
R0 for some viral diseases

Virus MERS coronavirus
Middle East respiratory syndrome coronavirus (MERS-CoV) virion – source CDC