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last updated: 10/24/05

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A submicroscopic organism that infects another organism's cells and can cause harm.  Viruses can be composed of DNA or RNA genetic material.

What are some examples of viruses?

Common cold, measles, chicken pox, flu, hepatitis, herpes, polio, …

What is a retrovirus?

A virus that stores its genetic information as RNA, but translates back to DNA before replicating.  This process is the reverse of the usual process and requires a special viral enzyme called Reverse Transcriptase.  HIV is one example of a retrovirus. 

What is the structure of HIV?
 
 

The HIV virus particle is composed of an envelope, outer surface glycoproteins, and an RNA genome

What is the immune system?

The body's defense against foreign invaders and cancerous cells.  It involves B cells that produce antibodies, T cells that directly attack foreign cells, and phagocytes that eat up foreign material.

How does HIV affect the immune system?

HIV infects helper T cells that display a certain protein, called the CD4 receptor.  Once inside the cell, HIV takes over the cell and the virus replicates.  In a couple of days, the cell dies and the new virus particles go on to infect more helper T cells.

What are the stages of the disease?

Stage 1 - Primary HIV infection - lasts a few weeks, flu-like symptoms
Stage 2 - Latent period - may last years, patient has no symptoms
Stage 3 - Symptomatic HIV infection - as more and more T cells are destroyed, 
               the body becomes overly susceptible to opportunistic infections and 
               cancers
Stage 4 - AIDS - helper T cell count is <200, patient develops 1 or more 
               opportunistic infections

What is an opportunistic infection? 

An illness that normal people with healthy immune systems can fight off.  People with AIDS cannot fight these infections and will eventually die.  Even the common flu can kill a person with AIDS.