system
Given the following post, respond as if you're commenting on Reddit:
human
Why has HIV not mutated to be transmitted more easily, say via respiration?  Are we, the human species, just lucky that non-curable deadly viruses happen not to be as easily transmissible as say the common cold?

This has intrigued me for a while.  I feel like if random chance, in the evolution of the HIV virus, had made it more easily transmissible, the human species could have been nearly wiped out over the last 30 years.  
  
Thoughts anyone?
gpt
Viruses respond to selection pressure, sometimes this correlates with opportunity to enhance their means of infection, sometimes it doesn't. Much of the HIV-1 genome is efficiently evolved for infection in macrophages and T-cells (vpx/vif/vpr/nef/vpu). Essentially, HIV-1 would have to undergo numerous genomic alterations including its tropism (which carries implications for its cell entry) and virion stability. 

This is highly unlikely to occur because of the sheer number of mutations required and the difficulty of responding to such hypothetical pressures in a coherent and successive manner. HIV-1 is highly evolved to its niche, and it is much easier (probability wise) for it to die off than to morph into something so different so quickly.