While your post is hypothetical, oddly enough, we actually have had somewhat similar events happen before to humans and know at least some consequences of such.

From about the mid-1500s to (at least) the mid-1600s disease went rampant among native populations on North America, killing potentially up to 90% of the population. Basically it was the disease apocalypse for many native peoples. In much of north american, in what used to be land used by people, curated, burned, hunted, fished, lived on, nature took back over. Plant and animals thrived and grew without humans. There's even thoughts this immense growth this contributed significantly to climate change at the time, with so much plant and animal life growing back that contributed in part to cooling the planet down.

When western explorers began exploring the american midwest and west, they were impressed by "virgin" areas of the world, untouched by man, nature growing untamed. But they weren't virgin at all it just happened to be all the people who used to live there died in the apocalypse a few hundred years before, and nature took back over.