Viruses are unable to reproduce or 'replicate' by themselves. They exist in two forms outside cells or inside cells. Outside cells they are called virions and have an outer 'envelope', which is similar to the membrane surrounding human cells. The envelope contains a limited number of viral proteins and its genetic material or 'genome'. The viral genome is made up of a few genes. HIV has nine genes called gag, pol, env, vif, vpu, vpr, tat, rev and nef. These determine the structure of viral proteins. The virus outside a cell cannot make the proteins itself. These are made inside cells using the host cell's capacity to produce proteins.

Viruses outside cells are divided into those which have the genome made of DNA, such as the herpes viruses, and those with the genome made of RNA, like HIV. RNA-containing viruses are known as 'retroviruses'. DNA and RNA are very similar except that slightly different chemicals form the building blocks in each type.