"One of the core reasons is the incredible rate of mutation that HIV goes through. What you see is that the rate of HIV mutation is such that HIV is distinct for each individual patient," explains Microsoft's Jonathan Carlson in the video above. You're probably thinking "Well, how many possible mutations could there be?"
Great question. The clip provides us with some basic, but very enlightening visualisations of just how insane HIV is compared to influenza. The first image shows the evolution of HIV, with samples taken from different patients. Then, a tiny orange dot is added to this image. That's influenza's evolution from a single outbreak. Then, another little coloured structure joins the first, showing the flu's mutations across the entire world. It is dwarfed by HIV.
Finally, a structure similar to the last appears — this is HIV mutations within a single patient.
It's fairly well known that HIV is a hard nut to crack, because it's constantly changing. But the scale and frequency of those changes? Well, this should put it into perspective.