With a cure for every disease, our evolutionary future might change. To understand why, we need to talk about genes. Genes are like options on a car. Alleles, which are sequences of DNA, make choices for those options. Option: Sunroof. Choice: Leaky sunroof. Among humans, there are plenty of alleles that are bad for us, as a leaky sunroof is on a car.
Some alleles are bad for us because they kill us or prevent us from having children when we're young. People who have such alleles often don't pass them on to their children. They die, or their disease prevents reproduction, so their alleles don't become very prevalent in the population. In our hypothetical world, those people would be cured before they were stopped from having children, so they would pass on their alleles to future generations.
Other alleles cause debilitating diseases, but not necessarily ones that prevent reproduction. In the real world, couples who carry alleles for these diseases might choose not to have children. In our what-if scenario, they might opt to become parents, since rather than suffering, those children could be cured.
Overall, disease-related alleles would become more common.
It's possible that even with a cure for every disease, little would change. People might not pursue them, even if they were available. Some people wouldn't know or would refuse to admit that they were sick. Others would avoid doctors, for whatever beliefs keep them away now. Another faction would get the wrong diagnosis or the wrong cure or reject treatment due to expense. Lastly, cures might have side effects, leading some folks to decide that the side effects were intolerable and choose to live with the disease.
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