A variety of invertebrates bite or feed on or in the human skin, including Flies, Fleas, Bedbugs, Lice, Mites, and Ticks. With a few exceptions (including larvae of a few flies, scabies mites and ticks) all these parasites bite, feed quickly, and leave. All of these skin parasites leave tell tale signs, including itchy, round, red papules (swellings). The majority of these bites will last about two weeks. If scratched (something that is hard to resist) the itchiness and swellings could last up to two months. In addition, human fingernails can be loaded with bacteria and scratching often leads to infections.

If symptoms of itching and crawling sensations in the skin persist and no evidence of parasites can be found, then a syndrome called Delusional Parasitosis must be considered. A variety of causes have been suggested for these sensations, including parasitism by Collembola and Strepsiptera or the presence of organisms called Morgellons. "Morgellons" is a term used to describe what are purported to be fiber-like parasites of the skin, but after decades of detailed study there is no evidence of an unknown organism fitting this description. There is also no evidence that Collembola or Strepsiptera are biologically capable of parasitzing humans. However, there are quite a number of physiological, hormonal and neurological syndromes that will cause these symptoms.

Information in this site specifically addresses parasites found in North America. The reason for the distinction is that there are other skin parasites, particularly in tropical regions that are not discussed in this site.