Hypersensitive

It was most obvious in the cities at first. With so many people in one area, it was a matter of statistics. At first everyone thought it must be a virus or something, but no dice. The reason for people swelling up, choking, dying? Allergies. People started developing allergies en masse, and no one knew why, let alone how to stop it.

Pretty soon it was everyone. Not all the allergies were severe, thankfully, but if you had eaten something, you were now allergic to it. Pretty soon hospitals were distributing food substitutes as fast as they could but for most people it was a matter of figuring out which foods wouldn't kill you immediately. Malnutrition can be nasty, but everyone thought that they just had to wait it out until doctors could figure out what was happening.

Unfortunately, the allergies weren't static. They just got worse and worse over time. People who thought it was safe to keep eating something because it just gave them a rash would be going into anaphylactic shock by the end of the week. It was possible to rotate between a few foods to stave off the inevitable, but only a handful of people had access to that much variety by the end.

Ultimately, though, what no one could deal with was the allergies to medicine. Anyone treated for allergies became allergic to allergy medication. Anyone who had prescription medicine had to face their conditions unmitigated. Even painkillers were now potentially deadly. That's when people gave up on the hospitals.

And that's how humanity finally died out. Surrounded by all the resources they could possibly need, they starved.