Spider lichen

Thought a relic of the past, spider lichen has regenerated after the human-free years of the plague

This lichen grows only in areas where certain common spiders have laid their webs to rest for a significant length of time.

Reproduction

Spider lichen spores can only germinate in the silken strands of spider webs. Once they have germinated, they coat the strand all the way to the wall, where they establish themselves. Once the lichen is properly established, the permanence of spiders, or their webs, is irrelevant to the species.

Gradual disappearance

The removal of spider webs from urban areas meant that the lichen had no means of establishing new specimens. Spider Lichen became less and less common in a gradual way so that its fading was not noticed until it was too late.
Although this was the main reason for its disappearance, it was not the only factor. Spider lichen was known to be good at repelling nightmares, and used to be brewed for children when they were going through periods of regular nightmares.

Re-discovery

As a consequence of the plague, many buildings were left empty and abandoned. Without anyone to take care of them, nature returned to them through cracks in the doors, broken shatters, and eventually, through collapsed roofs or rotten walls. Spiders proliferated throughout, and from the remnants of recondite forests, the spider lichen spores travelled until they eventually started settling in abandoned houses.
Not long after, lived-in houses were also colonized by spider lichen. Thankfully to the spiders and the cliche, during a pandemic, spiderwebs in the ceilings are not something most people pay attention to.

Perception

The return of spider lichen, brought back from the annals of history, was seen as a boon in the latter years of the plague, as people were trying to rebuild their lives.
The lichen is now being used to help people with Plague Mind have more calm dreams.