A virus is the smallest type of parasite to exist, usually ranging from 0.02 to 0.3μm in size, although some viruses can be as large as 1μm.
A viral particle or virion contains a single nucleic acid (RNA or DNA) core surrounded by a protein coat and sometimes enzymes that are required to initiate viral replication. Viruses can only replicate within the cells of animals, plants and bacteria and, as such, are referred to as obligate intracellular parasites.
Viruses are not classified according to the illnesses they cause; rather they are grouped into different families based on whether the nucleic acid is single- or double- stranded, whether a viral envelope is present and their mode of replication.
Rabies is a deadly viral disease of the nervous system, 3d illustration Credit: nobeastsofierce / Shutterstock
Single-stranded RNA viruses are further classified based on whether they have positive- or negative- sense RNA. DNA viruses tend to replicate within the nucleus of host cells, whereas RNA viruses generally do so in the cytoplasm.
Historically, few parasites have caused the devastation to animals, plants and humans that viruses have. Diseases such as polio, foot and mouth and smallpox are all well known for the widespread devastating effect on people and animals. Less well known about is the complete crop failure that a huge number of viruses have the potential to cause.
Are viruses alive?
When researchers first discovered viruses and realized they seemed to behave similarly to bacteria, they generally became considered as biologically "alive."
However, this changed in the 1930s when it was demonstrated that virions lacked the mechanisms that are required for metabolic function. Once scientists determined that viruses simply consist of DNA or RNA contained within a protein shell, they generally became thought of as biochemical mechanisms rather than living organisms.
A virus is typically made up of a protective protein coat called a capsid. Capsids vary in shape, from simple helical forms to more complicated structures with tails. The capsid protects the viral genome from the external environment and plays a role in receptor recognition, enabling the virus to bind to susceptible hosts and cells.
Sometimes the capsid is also contained within a phospholipid envelope derived from the membranes of the host cells it has infected. Viral encoded proteins called spike projections are usually found within this envelope. They are usually glycoproteins and they also help the virus move towards target cells via receptor recognition. One well known example is the influenza A virus, which expresses the glycoproteins neuraminidase and hemagglutinin on its surface.
The largest and most complex viruses can be viewed using a high-resolution light microscope.
Different types of viruses are different shapes, the two main ones being rods (or filaments), where the nucleic protein subunits are arranged in a linear fashion and spheres, which are icosahedral polygons.
The majority of plant viruses and many bacterial viruses are small filaments or polygons. Bacteriophages, which are larger, more complex and have double-stranded DNA are a combination of rod and sphere shapes. The well-known T4 bacteriophage has a polygonal head where DNA is contained and a rod-shaped tail made up of long fibers.
Bacteriophage T4 infecting some bacteria. Illustration Credit: Andrea Danti / Shutterstock
How do viruses infect?
Viruses do not have the mechanisms needed to survive independently and seek out plant, animal or bacterial host cells where they can use those cells’ machinery to replicate.
The virus enters hosts through horizontal or vertical transmission, mostly horizontal. Examples of horizontal transmission include the following:
- Direct contact transmission: This refers transmission via physical contact between an infected and uninfected subject through kissing, biting, or sexual intercourse, for example.
- Indirect transmission: Here, the virus is transmitted via contact with contaminated objects or materials such as medical equipment or shared eating utensils.
- Common vehicle transmission: This transmission mode refers to when individuals pick up the virus from food and water supplies that are contaminated with feces. This often causes epidemic disease.
- Airborne transmission refers to the respiratory infection that occurs when the virus is inhaled.
Once a virus has accessed its host, it recognises and binds to a specific receptor on the surface of a target cell. One well-studied example is the interaction that occurs between the CCR5 receptor on human T lymphocytes and the gp41 protein present on the surface of the human immunodeficiency virus (HIV).
HIV (AIDS) viruses in blood with red blood cells and white blood cells. 3D illustration Credit: Kateryna Kon / Shutterstock
Life cycle of a virus
Once a virus has infected a host cell, it can replicate within that cell thousands of times. Rather than dividing and reproducing in in the way that cells do, viruses go through a process called the lytic cycle.
First, the virus replicates its DNA and protein coats, which are then assembled into new virus particles. This causes the host cell to burst or “lyse,” which is why the cycle is so-called. The new virus particles that are released once the cell has burst then infect surrounding host cells.
The process can take as little as twelve hours, as is the case with the norovirus, or as long as several days, as is the case with the Ebola virus.
Some complex viruses called phages bind their DNA to that of their host cell or deposit small pieces of their DNA in the cytoplasm. When the cell then divides, the viral DNA is copied into the daughter cells. This cycle, which is called the lysogenic cycle, is less common than the lytic cycle.