Who...people with a bird's nest near the home, apartment dwellers with nesting pigeons, people with pets or other animals which are infested, farmers who raise chickens, people who obtain used furniture, carpet, clothing, etc, that has been infested. This can also include guests of hotels, office workers et al., when the buildings are inhabited by nesting birds.
What...parasitic infestation from bird mites too small to be easily seen without magnification. Symptoms include pinprick bites, often intense itching with or without lesions, small reddened bumps, and a crawling sensation anywhere on the body; with increased activity at night. Some people label these the "creepy crawlies" or "nose ticklers". The intense itching and irritation on the skin is due to the mite's saliva. When a large area is covered with bites it will resemble a rash in appearance, and it is often mistaken for scabies.