What is HIV? 

Outside of a human cell, HIV exists as roughly spherical particles (sometimes called virions). The surface of each particle is studded with lots of little spikes.  HIV particles are about 0.1 microns (one 10,000th of a millimeter) in size and although too small to be seen through an ordinary microscope, they can be seen clearly with an electron microscope. 

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HIV particles surround themselves with a coat of fatty material known as the viral envelope (or membrane).  Projecting from this are around 72 little spikes, which are formed from the proteins gp120 and gp41.  Just below the viral envelope is a layer called the matrix, which is made from the protein p17.

The viral core (or capsid) is usually bullet-shaped and is made from the protein p24.  Inside the core are three enzymes required for HIV replication called reverse transcriptase, integrase and protease.  Also held within the core is HIV's genetic material, which consists of two identical strands of ribonucleic acid (RNA), each containing a copy of the virus's nine genes. Three of these genes (gag, pol and env) contain information needed to make structural proteins for new virions.

HIV is a retrovirus, which means it has genes composed of RNA molecules.  Like all viruses, HIV replicates inside host cells.  It is considered a retrovirus because it uses an enzyme, reverse transcriptase, to convert RNA into DNA (deoxyribonucleic acid).