Sufferers of Morgellons disease complain of invisible parasites biting their skin. And they get skin lesions from which sprout strange fibers. And mysterious black spore-like specks appear on their skin. Cases of this strange disease seem to be spreading, especially in the Bay area. One theory is that it has something to do with Lyme disease. Or it may be a case of mass delusion. The medical community seems to think it's mass delusion. Most people who show up complaining of these symptoms get diagnosed with 'delusional parasitosis', which is a psychological problem in which people imagine that they're infested by parasites. Not having any medical qualifications at all, I won't weigh in on whether this is a real disease or mass delusion, but some of the behavior of the patients does sound suspiciously bizarre. Take the case of Theresa Blodgett:

She gathers up the black specks, the mysterious fibers and the small, fuzzy 'cocoons' she finds on her skin and around her home. She tapes the macabre samples to typing paper, but she said no doctor will analyze the collection. Physicians who glance at the specimens dismiss the lot as stray hairs, clothing fibers, scabs and other common household debris, she said.

So either she really is suffering from something and is desperately but unsuccessfully trying to get doctors to pay attention to her, or she's obsessively collecting house dust and stray flecks of dirt and convincing herself that these things are parasites attacking her. (Thanks to 'K' for the links)