by Jason Wasserman MD PhD FRCPC
March 9, 2022
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.
Although there are many different types of viruses (see below), most are made up of the same basic parts which include:
The combination of the genetic material surrounded by a capsule is called the viral particle.
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, it also cannot be killed. Instead, they are rendered inactive by products such as soap, alcohol, and bleach.
There are four ways for a virus to enter the human body: inhalation, oral ingestion, direct transfer, and through the skin. The way a virus enters the body will determine how it spreads, how quickly it spreads, and who is most likely to become infected.
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’s 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 affect the nose, throat, and lungs with symptoms that include nasal congestion, sore throat, cough, and difficulty breathing.
Diseases caused by viruses spread through droplets include COVID – 19, SARS, influenza, and the common cold.
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.
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.
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.
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 others 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”.
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.
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 affect 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.