Allergic

It was the trees. 

     In April of 2023, Neil Patterson made the discovery. But by then, the Earth had been nearly cleared of humans, the soil rich with their decomposition. 

     Neil had had a hunch. In full respiratory gear, he studied pollen taken from Deciduous and Conifer alike. Maple, Oak, Beech, Pine—all of the samples registered quantities of an allelopathic chemical once found only in leaf sap, a mild herbicide rained down as a means for clearing the soil immediately around the tree of competition for nutrients and water.
    In the pollen, however, Neil tested the chemical at nearly one hundred times the potency of what it had been in the sap. A single blossom, he found, had the capacity to inflict irreparable damage on human lung tissue. Springtime became a massacre.
     The aged fell first, the infirmed and young children.
  They thought it was a virus. Ran tests. Declared emergencies. Countries were quarantined. The dead couldn't be buried fast enough. Five years and Neil began to wonder if he had the Earth to himself. 

   
     From the quiet of his back porch, he watched now their gentle movements in the breeze, the sway of shadow over the unkempt grass. It was nearing fall. He was safe, for the moment, and smiled, remembering how helpless he had once thought they were, remembering the ambitious young tree-huggers he had known. He wondered if any of them had survived. If, now that the predator was removed, the chemical would weaken and return to the leaf sap—if the gift of clean oxygen the trees had once given freely, would become safe again to breath.
     Spring.
     Six months.
     He could only wait.