Viruses are tiny organisms that may lead to mild to severe illnesses in humans, animals and plants. This may include flu or a cold to something more life threatening like HIV/AIDS. 

How big are viruses?

The virus particles are 100 times smaller than a single bacteria cell. The bacterial cell alone is more than 10 times smaller than a human cell and a human cell is 10 times smaller than the diameter of a single human hair.

Are viruses alive?

Viruses by themselves are not alive. They cannot grow or multiply on their own and need to enter a human or animal cell and take over the cell to help them multiply. These viruses may also infect bacterial cells.

The virus particle or the virions attack the cell and take over its machinery to carry out their own life processes of multiplication and growth. An infected cell will produce viral particles instead of its usual products.

Structure of a virus

A virion (virus particle) has three main parts:

  • Nucleic acid this is the core of the virus with the DNA or RNA (deoxyribonucleic acid and ribonucleic acid respectively). The DNA or RNA holds all of the information for the virus and that makes it unique and helps it multiply.
  • Protein Coat (capsid) – This is covering over the nucleic acid that protects it.

Lipid membrane (envelope) – this covers the capsid. Many viruses do not have this envelope and are called naked viruses.

Genomic diversity among viruses
Property Parameters
Nucleic acid
  • DNA
  • RNA
  • Both DNA and RNA (at different stages in the life cycle)
  • Linear
  • Circular
  • Segmented
  • Single-stranded
  • Double-stranded
  • Double-stranded with regions of single-strandedness
  • Positive sense (+)
  • Negative sense (−)
  • Ambisense (+/−)

Courtesy:  Wikipedia


Viruses are not simply taken into cells. They must first attach to a receptor on the cell surface. Each virus has its specific receptor, usually a vital component of the cell surface. It is the distribution of these receptor molecules on host cells that determines the cell-preference of viruses. For example, the cold and flu virus prefers the mucus lining cells of the lungs and the airways.

How do viruses infect?

Viruses do not have the chemical machinery needed to survive on their own. They, thus seek out host cells in which they can multiply. These viruses enter the body from the environment or other individuals from soil to water to air via nose, mouth, or any breaks in the skin and seek a cell to infect.

A cold or flu virus for example will target cells that line the respiratory (i.e. the lungs) or digestive (i.e. the stomach) tracts. The HIV (human immunodeficiency virus) that causes AIDS attacks the T-cells (a type of white blood cell that fights infection and disease) of the immune system.

Life cycle of a basic virus

There are a few basic steps that all infecting viruses follow and these are called the lytic cycle. These include:

  1. A virus particle attaches to a host cell. This is called the process of adsorption
  2. The particle injects its DNA or RNA into the host cell called entry.
  3. The invading DNA or RNA takes over the cell and recruits the host’s enzymes
  4. The cellular enzymes start making new virus particles called replication
  5. The particles of the virus created by the cell come together to form new viruses. This is called assembly
  6. The newly formed viruses kill the cell so that they may break free and search for a new host cell. This is called release.

Host range

Viruses are by far the most abundant biological entities on Earth and they outnumber all the others put together. They infect all types of cellular life including animals, plants, bacteria and fungi. However, different types of viruses can infect only a limited range of hosts and many are species-specific. Some, such as smallpox virus for example, can infect only one species – in this case humans, and are said to have a narrow host range. Other viruses, such as rabies virus, can infect different species of mammals and are said to have a broad range. The viruses that infect plants are harmless to animals, and most viruses that infect other animals are harmless to humans. The host range of some bacteriophages is limited to a single strain of bacteria and they can be used to trace the source of outbreaks of infections by a method called phage typing.

Role in human disease 

Viral Infection

Overview of the main types of viral infection and the most notable species involved.

Examples of common human diseases caused by viruses include the common cold, influenza, chickenpox, and cold sores. Many serious diseases such as ebola, AIDS, avian influenza, and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence. Other diseases are under investigation to discover if they have a virus as the causative agent, such as the possible connection between human herpesvirus 6 (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome.[130] There is controversy over whether the bornavirus, previously thought to cause neurological diseases in horses, could be responsible for psychiatric illnesses in humans.

Viruses have different mechanisms by which they produce disease in an organism, which depends largely on the viral species. Mechanisms at the cellular level primarily include cell lysis, the breaking open and subsequent death of the cell. In multicellular organisms, if enough cells die, the whole organism will start to suffer the effects. Although viruses cause disruption of healthy homeostasis, resulting in disease, they may exist relatively harmlessly within an organism. An example would include the ability of the herpes simplex virus, which causes cold sores, to remain in a dormant state within the human body. This is called latency[132] and is a characteristic of the herpes viruses, including Epstein–Barr virus, which causes glandular fever, and varicella zoster virus, which causes chickenpox and shingles. Most people have been infected with at least one of these types of herpes virus. However, these latent viruses might sometimes be beneficial, as the presence of the virus can increase immunity against bacterial pathogens, such as Yersinia pestis.

Some viruses can cause lifelong or chronic infections, where the viruses continue to replicate in the body despite the host’s defense mechanisms.[135] This is common in hepatitis B virus and hepatitis C virus infections. People chronically infected are known as carriers, as they serve as reservoirs of infectious virus.[136] In populations with a high proportion of carriers, the disease is said to be endemic.