This is because HIV is a brilliantly designed virus (of course it evolved, it's not designed. I don't know a better way to say this though). It's a third-generation retrovirus, meaning it defeats internal cell safeguards in a very clever way, meaning that if one cell of a few types comes into contact with ~10 virus particles, you effectively have AIDS. For the flu we're talking hundreds of cells needing to be in contact with thousands of virus particles.

It evades being a target for vaccination by using internal body networks to transmit itself as opposed to the blood. The immune system inside our bodies has a sort of chemical protocol for one cell to exchange data with another. One cell approaches another, presents a specific molecule, and the cells link, exchange something (we don't know what, or why, or how for that matter), and the HIV virus slips along.

This means HIV looks at the immune system from behind all the detection mechanisms. You can look at immune cells as a mine : they have lots of traps on the outside. Trigger one of them and a massive army of huge, very angry, and very dangerous cells will overwhelm whatever triggered it (http://www.youtube.com/watch?v=JnlULOjUhSQ ), and a large part of things nearby that didn't trigger it too, usually. So where does HIV hide ? Simple : on the inside of the mine, transmitting itself to other mines when through the mechanic that comes by to repair the mine and doublecheck things every now and then.

There's other virii doing this, however they don't generally hide in the immune system. But for example Bof has the same tactic. It hides on the inside of cells, then every 3-4 months or so, one of the virus particles suddenly starts multiplying and makes it's host cell explode. The virus particles immediately get slaughtered by the immune system, but not before spreading the infection.

HIV is also mutating on purpose. Part of it's DNA (yes, the virus itself has RNA, however it's a direct analog. The virus, when in operation, consists of DNA though) is dedicated to mutate. It's got a long list of mutation instructions targeting specific parts of it's DNA, generally the ones referring to the virus mantle, but there are others. Effectively it contains instructions to copy it's RNA, then change the values at position x,y,z by random values from a specific range.

And HIV generally gets things right. It doesn't ever multiply fast enough to really set of alarm bells in the immune system. It avoids entering the blood stream except to spread from one human to the next(which is why even with regular sexual contact with an HIV positive individual, it takes ~6 months to get infected), and waits just long enough for the immune system to calm down before trying it again. It does this while still spreading fast enough to very slowly increase the number of infected people.

There are a number of countries on this planet, mostly in Africa and the middle east, where HIV has gone from being endemic to worse than that. In several places it is a matter of time until 100% of the population will be infected.

It is a very, very interesting virus.

A piece of good news is that, over time (but quite fast given that this is evolution we're talking about), HIV is becoming less lethal. This is exactly because it's infecting a significant fraction of living humans (without killing them). HIV is hardly the first virus to infect 100% of the human population, and there are some indications that other such viruses were somewhat similar. It also means that the evolutionary selection criteria for HIV will match those of the human race before long, and it should at some point mutate into a beneficial influence on the human race. The only way for HIV to spread it's genes will soon be to make more humans. At that point, it's no longer a virus but a part of humans, not much different from say, your liver, and it will find some function to fulfill. And again, it's hardly the first time this is happening to the human race, and it won't be the last.

yes this is not actually showing the cell type HIV attacks. However as far as I know there is no known video of the correct cell type actually doing something. Plus it's a very cool video.