Skip to content
    • Ico Human Biology Human biology
    • 16+
    • 45

Pathogens and the immune system

  of  10


Viruses range in size from 0.02µm to 0.3µm across, about 50 times smaller than the average bacterium. They are the smallest microorganisms. Viruses are not cells but they are a combination of genetic material and protein that can invade living cells. The viruses take over the biochemistry of the cells they invade to make more viruses. Since viruses can only reproduce as parasites in the cells of other living organisms, most scientists class them as obligate intracellular parasites.

The structure of viruses

Viruses are usually geometric shaped and have similar basic structures.

  • The capsid or protein coat is made up of simple repeating protein units known as capsomeres. This structure minimises the genetic material needed to code for coat production and simplifies assembling the protein coat in the host cell.
  • The genetic material is free within the virus. It may be DNA or RNA.
  • Some viruses have an outer lipid envelope, which is produced from the host cell. This makes it easier for the viruses to pass from cell to cell and may help them avoid the immune system.

Generalised structure of a virus

Classifying viruses

Viruses are classified by their genome and their mode of replication.

Type of virus Genetic material Mode of replication Examples
DNA viruses DNA Viral DNA acts directly as a template for both new viral DNA and for the mRNAs needed to synthesise viral proteins Adenoviruses that cause colds, bacteriophage that attack bacteria and varicella-zoster that causes chicken pox
RNA viruses RNA RNA acts as template for viral proteins either directly or indirectly Influenza virus, measles virus and tobacco mosaic virus
Retroviruses RNA Viral RNA controls production of reverse transcriptase. This makes DNA molecules corresponding to the viral RNA. They are transcribed to produce new viral proteins and a new viral genome. Human immunodeficiency virus (HIV)


Bacteriophage is an example of a DNA virus. The capsid of a bacteriophage is 70-90nm of diameter.
(Photo credit: Dr Graham Beards)

The measles virus is an RNA virus. Diameter of 120-250nm.
(Photo credit: CDC/Cynthia S.Goldsmith & William Bellini)

The human immunodeficiency virus (HIV) is a retrovirus. Diameter around 100nm.
(Photo credit: Wellcome Trust/ R.Dourmashkin).

Virus life cycles

As naturally occurring viruses invade and take over living cells to reproduce they all cause damage and disease of some sort. They can withstand drying and long periods of storage whilst maintaining their ability to infect cells.

How do viruses infect cells? They use several different methods:

  • Some viruses inject their genome into the host cell with most of the virus remaining outside
  • Some viruses are taken into the cell by endocytosis and the host digests the capsid, releasing the viral genome
  • The viral envelope may fuse with the host cell membrane releasing the rest of the virus into the cell
  • Plant viruses usually use a vector such as an aphid to get through the cell wall.

Chickenpox is caused by a virus
(Photo credit: CDC/NCIRD)

The life cycle depends on the type of virus. DNA viruses may go into a lysogenic pathway, when they steadily reproduce with the cells, or a lytic pathway, when they become virulent and cause disease.