Viruses are microscopic particles that exist almost everywhere on Earth. They are present in animals, plants, and other living organisms, and they can sometimes cause diseases.

Viruses are biological entities that can only thrive and multiply in a host, which is a living organism such as a human, an animal, or a plant. Some viruses cause disease. For example, severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2, causes the disease COVID-19.

A virus may also affect one organism in one way but a different one in another. This explains why a virus that causes illness in a cat may not affect a human.

Viruses vary in form and complexity. They consist of genetic material, DNA or RNA, with a coat of protein around it. Some have an additional coat called the envelope. This may be spiky and helps them latch onto and enter host cells. They can only replicate in a host.

In this article, we discuss in detail viruses, including how they act and how they can affect people.

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Electron microscope image of the first U.S. case of COVID-19. The spherical viral particles, colored blue, contain a cross-section of the viral genome, in the form of black dots.

Viruses are microscopic entities that have a core of genetic material, either DNA or RNA. The core is covered with a capsid, a protective coat made of protein.

Around the capsid, there may be a spiky covering known as the envelope. These spikes are proteins that enable viruses to bind to and enter host cells. There, if the conditions are right, they can multiply.

There is some dispute about whether viruses meet the criteria for living organisms. They can grow and reproduce, but they do not produce adenosine triphosphate, a compound that drives many processes in living cells.

They also do not contain ribosomes, so they cannot make proteins. This makes them unable to reproduce independently and totally dependent on their host.

After entering a host cell, a virus hijacks the cell by releasing its own genetic material and proteins into the host. It uses the host’s cellular machinery to make many copies of itself.

Next, the virus continues to reproduce, but it produces more viral protein and genetic material instead of the usual products that the cell would produce.

Viruses have different shapes and sizes. Scientists categorize viruses according to various factors, including:

  • their shape and size, which may be rod-shaped, almost spherical, or other shapes
  • the type of their nucleic acid, which contains their genetic information
  • whether or not they have a protective lipid envelope derived from the host cell

Examples of viruses with an envelope include the influenza virus and HIV.

Within these categories are different types of viruses. A coronavirus, for example, has a sphere-like shape and a helical capsid containing RNA. It also has an envelope with crown-like spikes on its surface.

Seven coronaviruses can affect humans, but each one can change or mutate, producing many variants.

Learn more about coronaviruses here.

What are friendly viruses?

Just as there are friendly bacteria in the intestines that are essential to gut health, humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli.

Viruses do not leave fossil remains, so they are difficult to trace through time. Scientists use molecular techniques to compare the DNA and RNA of viruses and find out more about where they come from.

Three competing theories try to explain the origin of viruses. In reality, viruses may have evolved in any of these ways.

The regressive, or reduction, hypothesis suggests that viruses started as independent biological entities that became parasites. Over time, they shed genes that did not help them parasitize, and became entirely dependent on the cells they inhabit.

The progressive, or escape, hypothesis postulates that viruses evolved from sections of DNA or RNA that “escaped” from the genes of larger entities. In this way, they gained the ability to become independent and move between cells.

The virus-first hypothesis suggests that viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells on Earth appeared, billions of years ago.

When a viral disease emerges, it is not always clear where it comes from. For instance, experts believe SARS-CoV-2 originated in bats and then spilled over into humans.

A virus exists only to reproduce. When it reproduces, particles spread to new cells and new hosts. The features of a virus affect its ability to spread.

Viruses can spread through:

  • Touch: If, for example, a person has the SARS-CoV-2 virus on their hands, and they touch their nose, mouth, or eyes, the virus can enter the body, and they can develop COVID-19.
  • Respiratory droplets: Some viruses can be present in respiratory droplets. A person produces these when they talk, cough, or sneeze. Influenza and SARS-CoV-2 are two examples of viruses that can spread in this way.
  • Direct contact: Some viruses may spread through direct contact with a person that has the virus. For example, the human papillomavirus (HPV) can spread via direct contact with the skin. The Epstein-Barr virus, which causes mononucleosis (mono), can spread through saliva, such as while kissing.
  • Bodily fluids: HIV, for instance, can pass from one person to another through the exchange of semen or blood.
  • Contaminated food or water: Noroviruses are one type of virus that can enter the body when a person consumes contaminated food or water.
  • Insects: Mosquitoes carry the virus that causes Zika from one person to another.
  • Around childbirth: A mother with the cytomegalovirus, which is a herpes virus, can pass the virus on to an unborn child.

Some viruses can remain active on an object for some time. If a person with the virus on their hands touches an item, the next person can pick up that virus by touching the same object. The object is known as a fomite.

Viruses often change over time. As they reproduce, “copying errors” and genetic changes naturally occur. Some of these changes are very small and do not cause concern, but others can be more significant.

Significant changes could make a virus more transmissible, as has been the case with the B.1.1.7 variant of SARS-CoV-2.

They may also help the virus evade the immune system or existing treatments. For example, doctors use several drugs in combination to treat HIV so that it is harder for the virus to develop resistance to treatment.

Influenza viruses can also do so-called antigenic shift. This can happen if a host cell has become infected with two different types of influenza virus. The two influenza viruses can “reassort” to produce a novel influenza virus. For instance, pigs can often serve as a mixing vessel for avian and human influenza viruses.

Viruses cause many human diseases. For example, the Epstein-Barr virus can lead to mono.

Other viral diseases include:

Some viruses, such as HPV, can lead to cancer.

The full impact of a virus can take time to appear, and sometimes there may be a secondary effect.

For example, the herpes zoster virus can cause chickenpox. The person recovers, but the virus may stay in the body. Years later, it may cause shingles in the same individual.

SARS-CoV-2, the virus that causes the disease COVID-19, is a coronavirus. Coronaviruses are a large family of viruses and include viruses that cause the common cold.

Overall, experts consider SARS-CoV-2 a relatively stable virus. However, it has changed many times since scientists first identified it in China.

By September 2020, scientists had logged over 12,000 mutations, and the development continues.

Some variants are more transmissible and more likely to cause severe illness than others. The main concern with new variants is the unpredictability of their impact.

There may also be uncertainty about how well current vaccines can combat a new variant.

The main symptoms of COVID-19 are dry cough, fatigue, and fever, but there are many possible symptoms.

Anyone who has symptoms should seek a test. It is also important to self-isolate until 10 days after symptoms appear and when no fever has been present for 24 hours.

If a person has difficulty breathing, they should seek emergency medical attention.

When the body’s immune system detects a virus, it starts taking measures to protect the body.

As viruses enter the body’s cells, the immune system cannot “see” the virus. However, special T cells, known as cytotoxic T cells, can recognize cells that contain viruses, and release substances that kill those cells.

Some viruses can escape detection by cytotoxic T cells, but other immune cells — natural killer cells — can cause the cell containing the virus to die.

In addition, body cells that contain a virus emit proteins called interferons, which warn other cells that a virus is present. This gives healthy cells a chance to defend themselves by changing the molecular makeup of their surface.

Antibodies can also help fight a virus before it enters a cell. They do this by neutralizing or damaging the virus or by changing its features so that it can no longer enter healthy cells.

People may have antibodies if they have already had a virus or if they have received a vaccine.

Antibiotics treat bacterial infections, but they cannot treat a viral infection. People will need either a vaccination to prevent infection, or antiviral drugs to treat any symptoms. Sometimes, the only option is symptom relief.

In recent decades, scientists have developed antiviral drugs, largely in response to the AIDS pandemic. These drugs do not destroy the virus, but they slow or prevent its development.

With antiviral treatment for HIV, for example, the level of virus in the body can become so low that tests cannot detect it. At this point, it becomes untransmittable, which means that a person cannot pass the virus on to another person.

Antivirals are also available to treat infection with HSV, hepatitis B, hepatitis C, influenza, shingles, and chickenpox.

Tamiflu is an example of an antiviral drug. People can use it to manage influenza.

Vaccination can be an effective way of preventing viruses from causing disease.

Some vaccines have succeeded in eliminating diseases such as smallpox, which experts believe has been around for at least 3,000 years.

Here are some ways a virus vaccination may work:

  • It contains an inactivated form of the virus.
  • It contains a live attenuated virus, as in immunization for polio.
  • It is an mRNA vaccine, which teaches the body to make proteins that fight a specific virus, such as SARS-CoV-2.
  • It uses viral vector technology to create a modified version of a virus such as Ebola or SARS-CoV-2. The vaccine does not contain the real virus but teaches the body to fight the actual virus.

Currently, vaccinations exist for polio, measles, mumps, rubella, COVID-19, and various forms of the flu, among other conditions.

Vaccination can dramatically reduce the likelihood of becoming seriously ill due to a virus, as well as the risk of passing a virus on to others.

For example, according to the Centers for Disease Control and Prevention (CDC), two doses of the measles vaccine offer 97% protection from measles, a highly contagious and potentially fatal condition.

Moreover, widespread use of the vaccine has reduced the incidence of measles in the United States by 99% since it first appeared. If there is an outbreak, it usually affects people who have not had the vaccine.

However, if fewer than 92–95% percent of people receive the vaccine, a community can lose its herd immunity. As a result, the risk of disease increases dramatically, and an outbreak can occur.

Research shows that COVID-19 vaccines are safe and effective at preventing serious illness in those exposed to the SARS-CoV-2 virus. Health experts encourage people to have this vaccine to protect themselves and others.

Viruses are biological entities that are present in all living beings. Some are harmless, while others can cause a range of diseases, from the common cold to Ebola.

Seeking protection from potentially hazardous viruses — for example, through vaccinations — can help prevent serious illness.