A new strain of the flu, say H3N2 (which can, incidentally, infect swine, birds, and humans) turns out to be "the big one" that sweeps the world like the plague. It doesn't show many symptoms until well after it's communicable, and hits an individual so quickly afterwards that even with good medical care there's a good chance of death.
While scientists are busy building a vaccine for it, epidemiologists note that those infected with HIV are almost 100% immune to the effects of this new flu, though the mechanism is unknown. Africa and other parts of the world with an HIV infection rate of 20% or more are impacted, but not nearly as heavily as more developed parts of the world. It turns out to be an illness of the wealthy.
The movie follows the lives of several people, connected by fate, as they face the this plague. One who is involved in the research and must make hard decisions on what to recommend, particularly in terms of quarantine since there's no easy way to contain a virus that shows few symptoms early. A family who see people around them getting sick, and choosing, or not, to trade chance of death due to flu for lifetime infection with HIV (a choice made more difficult as they must decide for their children who are more likely to die from the flu). A politician trying to unseat the backwards incumbent that is bent on old out of date policy doomed to kill most of his constituents in order to reap the benefits of his investments in advanced medical care. A man who's wife was pregnant when she was infected, is now brain dead due to fever because she refused drugs that would have put the baby in danger, but is breathing on her own, responding well to the feeding tube, and the baby is probably safe due to the placental barrier, but the delivery is risky, HIV infection is risky, and the status of the baby's possible infection is unknown because any test that would prove it breaks the barrier, etc. In the initial fog of war it's unclear how clean things must be in order to prevent infection, and people are reluctant to visit the hospital, urgent care centers, or doctors as they believe they will get it while there. Lots of people fleeing urban centers, only to find that they are infecting suburbia. Doomsday preppers take to the hills or their bunkers.
Lots of stories to tell. Overall the flu becomes yet another flu the human immune system can manage for those that survived, though a staggering portion of the population chose HIV, which, ironically, leads to more profit motive and eventually an actual cure, not just management, for HIV due to the rate of infection. Scientists also discover the mechanism that HIV used which caused the virus to fail, and have created a medicine that duplicates that functionality, thus curing most variants of influenza in the human race altogether, for those in developed countries anyway. While it takes time, HIV and influenza are both expected to to be put on the shelf of history alongside polio, diphtheria, whooping couch, and others.
While billions died, the human race has eradicated two diseases, and are overall better off than they were prior to the infection. A basic, "whatever doesn't kill you makes you stronger" theme.
You could replace HIV with HPV, or a communicable form of cancer, or something else, and the main theme would stay intact, though HIV is unique in many ways that makes it a particularly interesting target.
Could be an action thriller, which the movie studios like, but it would probably be more interesting as a thoughtful drama, though not as profitable.
