Unlike most pathogenetic organisms viruses only true characteristic of a living organism is its ability to reproduce. They do not respire, excrete, grow and they are not sensitive.

Viruses are obligate parasites, this means that they can only reproduce from within a host cell. All animals, plants and even bacteria are susceptible to viruses. Viruses are always associated with diseases since they destroy the host cells. It must be noted that viruses are specific to their host cell.

Viruses are very small (about 20-300 nanometers) and are therefore easily spread. Viruses are spread and contracted through different means:
  • Respiratory viruses are spread through mainly coughing and sneezing.
  • Poliomyelitis is spread through faeces and flies.
  • HIV is passed through sexual contact and blood transfusion. 
Viruses are not cells, they contain either viral RNA or viral DNA surrounded by a protein capsid. A virus carries the basic needs, it has all the components apart from the ability to reproduce on its own.

The Invasion of a cell

A virus may invade a cell through either budding or through injecting its viral DNA/RNA into the cell. The virus then takes over and alters the cell's instructions. It uses the cell's ATP, nucleotides and amino acids to create new viruses. Once the viral DNA/RNA has been replicated and capsids created, virus numbers increase, this either causes budding to reoccur and for the virus to escape or cell lysis occurs.

As you now know, a virus may contain RNA, if it does, this means that it is a retrovirus. Since the virus is unable to transcribe its DNA then it has an alternate method, as well as containing RNA, it also has reverse transcriptase. This brings about the reverse of normal transcription and allows it to replicate.

Acquired immune deficiency syndrome (AIDS)

AIDS is caused by the retrovirus - human immunodeficiency virus (HIV). HIV is slightly different as it attacks the helper T cells and becomes attached by glycoprotein to the helper T's surface. Once the HIV has replicated it is known to lie dormant for years before it buds of and attacks more helper T cells. Due to the decrease in number of helper T cells, it gives opportunistic infections such as rare forms of cancer to attack the body. There is no cure for HIV and AIDS as its antigen changes frequently.