News agencies are reporting the supposed success of medical trials of vaccines against HIV (human immunodeficiency virus), the predecessor of AIDS, in Thailand. The road up to this point has been a long one, from the callous indifference of the general public towards the AIDS outbreak in the mid 1980's to early 1990's, to the massive activism and fund raising efforts we see today as the gravity of the situation has dawned on world leaders and relief organizations. But why has it been so hard to create a vaccine with the potential to prevent HIV infection and subsequently progression into AIDS? And why are effective treatments of these diseases so incredibly difficult to achieve?

The answer lies in the basic biology of the HIV/AIDS virus and the human immune system.

Principles of Vaccination

The idea behind vaccinations is the if you expose your immune system to a foreign substance in your body (virus, bacteria, fungus, etc.), given enough time your body will recognize it as foreign and your immune system will work to clear out the invading infection.  When you are exposed again to the same type of foreign substance, your immune system has special cells that "remember" this substance and will help clear it out more quickly. 

Vaccines work by exposing a person to a virus or bacteria in a weakened or killed form.  That way, the person receiving the vaccination does not get the disease or the symptoms but will still gain immunity from this exposure.  This is the same principle that causes you to only get chickenpox once- you have likely been exposed more than once, but you will (hopefully) only get those awful itchy blisters once in your life because your body will remember the virus. 

Viruses like the flu or the common cold change seasonally and very quickly, which is why no matter how hard we try, we are unlikely to come up with an effective vaccine against the common cold.  It takes months to develop and produce enough of an effective vaccine; by the time the vaccine would be available to the public, the virus would have changed.  Think of it as a kids cartoon: the hero (your immune system) is trying desperately to pick the villain (the virus) out of a crowd (your body).  But just when the hero thinks he has the villain cornered (i.e. administering a vaccine), the villain slips out a side door and changes his disguise.  It's a continuous game of cat and mouse.

Viruses are especially difficult to treat because they cannot live on their own- they are essentially non-living entities.  They must invade a healthy cell in order to reproduce.  The reason we have such a hard time curing viruses is because it is difficult to target the virus specifically while it is living inside of your own cells, without killing an excessive number of healthy cells in the process.

So what?

Remember the part about your body remembering the virus?  Once you've been exposed, you retain some level of immunity in special immune cells in your body.  The HIV/AIDS virus has evolved to use this principle to its advantage.

Different viruses specialize on different cells.  Flu viruses infect your respiratory system, so you get a runny nose and cough as the virus infects and kills cells in this area.  Chickenpox infects your skin and nerve cells, so you get a rash as the virus infects and kills these cells.  HIV specializes on immune cells- the cells that are supposed to be clearing the virus out of your system!  HIV injects itself into the cells that attempt to clear the virus out of the body- and then it waits.

When another foreign invader is detected within the body, the immune cells responsibly for the "memory" of the immune system multiply very rapidly to aid in clearing the infection.  While these cells are multiplying, if they have been infected by the HIV virus, the virus may be multiplying within the cells themselves.  This results in the immune system cells dying off, like the cells in your lungs when you have the flu (this iswhy you get a sore throat and cough).  Instead of a cough or sore throat though, HIV causes depressed immune function.

Because HIV invades the cells that would be used to 'remember' exposure to it, a vaccine has been incredibly difficult to develop.  Exposing a person to the virus would likely just result in an infection, and this is obviously an unsatisfactory outcome in a clinical trial.

In conclusion...

For those of you who have stumbled through this attempted non-technical explanation of a very technical topic, let us all hope and pray that the breakthroughs being reported in Thailand in the field of AIDS and HIV are indeed the first step towards eliminating this disease around the world.