As the first biological entities emerged out of the water and into the land, the battle was just beginning. The toxic air was oxygenated over time and as complexity increased, they had to develop sophisticated systems to breathe. The toxin turned into life-giving Oxygen and over a million years, humanoids experimented with systems that could shuttle the magic molecule to power their systems.
Allergies and asthma followed modern humans as they transversed the globe with their badly designed respiratory systems, prone to catastrophic failure and that killed them in large numbers. Later, modern medicine will keep them alive for a few more decades but they often succumb to the inability to oxygenate. Their nemesis, the Virus, attacked their Achilles heel as most died of the common cold and some of the more advanced versions, aptly named Pneumonia. Recent news (1) that claims advances in bronchodilator drugs in asthma is certainly welcome news.
Humans, fragile and badly designed in most systems, do not appear to be robust enough to move to the next stage.