@RaimundoBruno—
It seems rather clear.
A girl had mushrooms growing from her head, and was rejected by others in her age-group; some of them treated her as if her condition were contagious. She retreated to her home and stayed there. One of the mushrooms grew especially larger. When she had her head out a window, the wind caused that mushroom to loft her into the air, rather like the umbrella of Mary Poppins. Children watched in wonder, not noticing that the mushroom were distributing its spores, some of which fell on each of them. Mushrooms began to grow from their heads as well. Her condition was, indeed, contagious, though perhaps only became so as a result of how they had treated her.