Viruses are microscopic organisms that exist almost everywhere on earth. They can infect animals, plants, fungi, and even bacteria.

Sometimes a virus can cause a disease so deadly that it is fatal. Other viral infections trigger no noticeable reaction.

A virus may also have one effect on one type of organism, but a different effect on another. This explains how a virus that affects a cat may not affect a dog.

Viruses vary in complexity. They consist of genetic material, RNA or DNA, surrounded by a coat of protein, lipid (fat), or glycoprotein. Viruses cannot replicate without a host, so they are classified as parasitic.

They are considered the most abundant biological entity on the planet.

Fast facts on viruses

Here are some key points about viruses. More detail is in the main article.

  • Viruses are living organisms that cannot replicate without a host cell.
  • They are considered the most abundant biological entity on the planet.
  • Diseases caused by viruses include rabies, herpes, and Ebola.
  • There is no cure for a virus, but vaccination can prevent them from spreading.

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Effects of viruses can range from life-threatening to virtually symptomless.

Almost every ecosystem on Earth contains viruses.

Before entering a cell, viruses exist in a form known as virions.

During this phase, they are roughly one-hundredth the size of a bacterium and consist of two or three distinct parts:

  • genetic material, either DNA or RNA
  • a protein coat, or capsid, which protects the genetic information
  • a lipid envelope is sometimes present around the protein coat when the virus is outside of the cell

Viruses do not contain a ribosome, so they cannot make proteins. This makes them totally dependent on their host. They are the only type of microorganism that cannot reproduce without a host cell.

After contacting a host cell, a virus will insert genetic material into the host and take over that host's functions.

After infecting the cell, the virus continues to reproduce, but it produces more viral protein and genetic material instead of the usual cellular products.

It is this process that earns viruses the classification of parasite.

Viruses have different shapes and sizes, and they can be categorized by their shapes.

These may be:

  • Helical: The tobacco mosaic virus has a helix shape.
  • Icosahedral, near-spherical viruses: Most animal viruses are like this.
  • Envelope: Some viruses cover themselves with a modified section of cell membrane, creating a protective lipid envelope. These include the influenza virus and HIV.

Other shapes are possible, including nonstandard shapes that combine both helical and icosahedral forms.

Viruses do not leave fossil remains, so they are difficult to trace through time. Molecular techniques are used 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.

  • Regressive, or reduction hypothesis: Viruses started as independent organisms that became parasites. Over time, they shed genes that did not help them parasitize, and they became entirely dependent on the cells they inhabit.
  • Progressive, or escape hypothesis: Viruses evolved from sections of DNA or RNA that "escaped" from the genes of larger organisms. In this way, they gained the ability to become independent and move between cells.
  • Virus-first hypothesis: Viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells appeared on Earth, billions of years ago

A virus exists only to reproduce. When it reproduces, its offspring spread to new cells and new hosts.

The makeup of a virus affects its ability to spread.

Viruses may transmit from person to person, and from mother to child during pregnancy or delivery.

They can spread through:

  • touch
  • exchanges of saliva, coughing, or sneezing
  • sexual contact
  • contaminated food or water
  • insects that carry them from one person to another

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

As the virus replicates in the body, it starts to affect the host. After a period known as the incubation period, symptoms may start to show.

What happens if viruses change?

When a virus spreads, it can pick up some of its host's DNA and take it to another cell or organism.

If the virus enters the host's DNA, it can affect the wider genome by moving around a chromosome or to a new chromosome.

This can have long-term effects on a person. In humans, it may explain the development of hemophilia and muscular dystrophy.

This interaction with host DNA can also cause viruses to change.

Some viruses only affect one type of being, say, birds. If a virus that normally affects birds does by chance enter a human, and if it picks up some human DNA, this can produce a new type of virus that may be more likely to affect humans in future.

This is why scientists are concerned about rare viruses that spread from animals to people.

Viruses cause many human diseases.

These include:

Some viruses, such as the human papilloma virus (HPV), can lead to cancer.

What are friendly viruses?

Just as there are friendly bacteria that exist in our intestines and help us digest food, humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli (E. coli).

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The body defends itself through the deployment of T-cells, which attack the virus.

When the body's immune system detects a virus, it starts to respond, to enable cells to survive the attack.

A process called RNA interference breaks down the viral genetic material.

The immune system produces special antibodies that can bind to viruses, making them non-infectious. The body sends T cells to destroy the virus.

Most viral infections trigger a protective response from the immune system, but viruses such as HIV and neurotropic viruses have ways of evading the immune system's defenses.

Neurotropic viruses infect nerve cells. They are responsible for diseases such as polio, rabies, mumps, and measles.

They can affect the structure of the central nervous system (CNS) with delayed and progressive effects that can be severe.

Treatment and drugs

Bacterial infections can be treated with antibiotics, but viral infections require either vaccinations to prevent them in the first place or antiviral drugs to treat them.

Sometimes, the only possible treatment is to provide symptom relief.

Antiviral drugs have been developed largely in response to the AIDS pandemic. These drugs do not destroy the pathogen, but they inhibit their development and slow down the progress of the disease.

Antivirals are also available to treat infection with the herpes simplex virus, hepatitis B, hepatitis C, influenza, shingles, and chicken pox.

Vaccines

Vaccinations are generally the cheapest and most effective way to prevent viruses. Some vaccines have succeeded in eliminating diseases, such as smallpox.

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Vaccination is the most effective way to prevent viruses.

Virus vaccinations consist of:

  • a weakened form of the virus
  • viral proteins called antigens, which stimulate the body to form antibodies that will fight off future
  • infections with the same virus
  • live-attenuated viruses, such as immunization for poliomyelitis

Live-attenuated vaccines carry the risk of causing the original disease in people with weak immune systems.

Currently, vaccinations exist for polio, measles, mumps, and rubella, among others. Widespread use of these vaccines has reduced their prevalence dramatically.

Two doses of the measles vaccine, for example, offer 97 percent protection against this disease.

The measles vaccine has achieved a 99-percent reduction in the incidence of measles in the United States (U.S.). If there is an outbreak, it usually affects people who are not vaccinated.

Some people choose not to vaccinate their children, and because most people around them do vaccinate, the risk of getting measles is low.

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

In the words of the CDC:

"Antivaxxers help breathe new life into old diseases."

This can also affect vulnerable people who are unable to receive the vaccine for some reason, such as a compromised immune system.

Viral infections usually resolve without treatment, but medication can relieve symptoms such as pain, fever, and cough.