There are many diseases that have been basically eradicated from rich countries because they are easy to cure with modern medicine, but they persist in poor nations for the lack of money for basic treatments. For example, I’ll be taking students to Guatemala next semester and the CDC recommends getting the typhoid vaccine for travelers to rural parts of Guatemala. It is really cheap to treat typhoid with antibiotics and prevent it with sanitation, and that is why typhoid was basically eradicated in the USA decades ago, but it keeps getting reintroduced every year by travelers who pick it up in poor countries.
If every country were rich, Typhoid would probably be eradicated already because it only lives in human hosts. As would cholera, leprosy, hookworm, dracunculiasis and many other parasites and diseases that middle-class people can easily afford to cure. In a world where everyone has a middle-class income, many infectious diseases would have been eradicated by the ordinary efforts of middle-class individuals to avoid the disease. These diseases have only survived due to reservoirs of poverty where they have found refuge. Without poverty there would be no need for a decades-long Global Polio Eradication campaign. Middle class people would have eradicated it on their own initiative.
Middle-class society eradicated the bedbug mostly through individual efforts. It disappeared from large swathes of the world for decades, but it survived in places of poverty where it became resistant to insecticides and now it has bounced back into the beds of hotels of every income level around the world. If everywhere had had a middle-class income during the insecticide era, the bedbug would be extinct today, and probably head lice too. Middle-class areas are so good at eliminating malaria, I’d bet that it would have been eradicated long ago too if everywhere were middle class. What other diseases would have been inadvertently eradicated by individual actions if everyone in the world had been middle class?