Pathology dictionary

What is a virus?

Virus particle

A virus is an incredibly small biological agent capable of infecting animals, plants, bacteria, and fungi. The typical virus is approximately 1/10 the size of a single bacteria and 1/100 to 1/1000 the size of a human cell.

Virus vs bacteria vs human cell

Although there are many different types of viruses (see below), most are made up of the same basic parts which include:

  • Genetic material – This can be in the form of DNA or RNA. Human cells also contain genetic material in the form of DNA. RNA is a special type of genetic material that can be used to quickly make proteins.
  • Capsule – The genetic material is surrounded by a capsule made out of specialized virus proteins.
  • Envelope – Some viruses have an extra layer outside of their capsule called an envelope. The envelop is made out of fat (lipids) and specialized virus proteins. Another name for the envelope is membrane.

The combination of the genetic material surrounded by a capsule is called the viral particle.

Parts of a virus

Is a virus alive?

Most scientists do not consider viruses to be alive. That is because they are not able to reproduce or carry on the normal functions of life (metabolism) without first infecting another organism that is alive (such as a bacteria, plant, or animal). However, some people believe that because viruses have their own genetic material, they should be considered a simple (and very small) form of life. Because a virus is not technically alive, they also cannot be killed. Instead, they are rendered inactive by products such as soap, alcohol, and bleach.

How do viruses enter the body?

There are four ways for a virus to enter the human body. The way a virus enters the body will determine how it spreads, how quickly it spreads, and who is most likely to become infected.

Inhaled droplets

Droplets are produced when people who are already infected with the virus and contagious cough or sneeze. Once in the air, the virus filled droplets can enter another person body through their nose or mouth. Droplets can also land on the surface of an object and be transferred to the nose, mouth, or eyes by hand.

Viruses that spread through droplets often effect the nose, throat, and lungs with symptoms that include nasal congestion, sore throat, cough, and difficultly breathing.

Diseases caused by viruses spread through droplets include COVID – 19, SARS, influenza, and the common cold.

Food or water

Humans can become infected by a virus in contaminated food or water. The virus enters the body through the stomach or bowels when the contaminated food or water is swallowed. Viruses spread through food or water often affect the gastrointestinal tract and cause symptoms such as nausea, vomiting, and diarrhea.

Diseases caused by viruses in contaminated food or water include viral gastroenteritis and hepatitis.

Direct transfer

Some viruses need to move directly from one person to another in order to spread. These viruses are typically spread by blood, sexual contact, or from mother to child at the time of birth.

Diseases caused by viruses spread by direct transfer include hepatitis, HIV, and herpes.

Bites from insects

Some viruses are spread by insects. The insects are called carriers because they carry viruses inside their body but are not harmed by the virus. Humans become infected when they are bitten by an insect carrying the virus. Most of the viruses in this group are found in warm climates where insects are found all year long.

Diseases caused by viruses spread by bites from insects include West Nile disease and yellow fever.

What happens after a cell is infected by a virus?

Once a virus enters the body, it needs to get inside of a cell before it can create new copies of itself and spread. The types of proteins on the capsule or envelope determine which types of cells the virus can enter. For example, some viruses can only enter cells in the respiratory tract while other can only enter cells in the gastrointestinal tract. The ability of a virus to attach and enter only certain types of cells is called “tropism”.

  • Attachment – Once the virus gets into the body and finds its target cell, it attaches itself to the surface of the cell using the specialized proteins found on the capsule or envelope. The viral proteins stick to another type of protein on the outside surface of the cell called a receptor. The types of cells that make a receptor determine the tropism of a virus.
  • Entry – After the virus sticks to the receptor on the surface of the cell, it is brought inside the body of the cell and is released from the receptor.
  • Replication – Once inside the cell, the virus uses the machinery of the cell (the proteins normally found inside the cell) to create new viral genetic material and specialized viral proteins. A virus must be inside a cell to make new genetic material or proteins.
  • Assembly – New viral particles are then made out of the genetic material and proteins made during the replication step. A cell infected by a single virus can produce thousands of new viral particles.
  • Release – Once the new viral particles are assembled, they need to leave the cell so that they can infect other cells. Some viruses leave the cell by causing the cell to explode. This kills the cell while releasing all of the virus particles at the same time. Viruses that require an envelope attach to the wall of the cell (the membrane) and take some of the wall with them as they leave the cell. This is called budding. Some viruses can remain in a cell for months or even years before producing new virus and leaving the cell.

Cycle of a virus

Are there different types of viruses?

Yes, there are many different types of viruses and they are organized into groups called families. The members of a virus family share genetic material (just like members of a human family share DNA)

The table below lists some of the most common virus families and the diseases associated with those families.

virus family table

What does it mean when people say someone is contagious?

A person who is contagious is infected with a virus and able to spread the virus to other people. In order for this to happen, the infected person’s body must be making and releasing new viral particles. When this occurs in the course of a disease is depends on the type of virus being produced.

For example, most viruses that effect the nose, throat, and lungs (respiratory tract) cause a person to be highly contagious for a relatively short period of time at the beginning of the disease. In contrast, many blood borne or direct contact viruses can stay in the body and cause the person to be contagious for many years.

A common misconception is that a person needs to be showing symptoms of the disease in order to be contagious. This is not true. For many types of viruses, the infected person is contagious well before symptoms start.

The opposite is also true. For many types of viruses, a person stops being contagious before the symptoms go away fully. That is because the body continues to show signs of the disease well after the last virus has been inactivated.

Other helpful resources

Center for Disease Control and Prevention

World Health Organization

Coronavirus Resource Centre – Harvard Health

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