Though the term may be inaccessible to many, "erythema" is patently simple to understand, and usually, rather plain to see. This much is understandable, though, given it is the skin that is the canvas for erythema's expression, so to speak. The erythemas, technically, are not diseases but rather signs of a underlying cause or disease, the true nature of which is, as a matter of fact, not well comprehended by doctors and scientists alike. Thus, for the most part, these conditions are not a serious danger themselves, and in light of their eruption, should not provoke worry in afflicted individuals. These people and their physicians should still take note, however, for erythema in its various forms may be used as a diagnostic tool. 

Background

Erythema multiforme, the generalized designation given to erythema, is characterized by reddened portions of the skin that appear all over the body. The exact mechanism which drives erythema multiform is unknown, but it is seen more often in those who suffer from drug allergies and infections. 

Erythema Nodosum

Erythema nodosum tends to be restricted more to the shins and other areas below the knees, and internally, is formed by nodules within the leg's fatty tissue. Common co-morbid conditions include colitis and streptococcus. If the underlying cause is known, it can be treated accordingly with antibiotics or other measures.

Erythema Migrans

Rather than having multiple potential sources, erythema migrans is indicative of Lyme disease. While recognition of erythema migrans is, for this matter, a discovery not to be taken lightly, as the tick-borne disease must be treated, its presentation in Lyme patients occurs more often than not and early at that.  

Palmar Erythema

So-named for its localization to the palm, more specifically the lower inner small finger and thumb, palmar erythema is an abnormal coloration of the skin without inflammation. It coincides with a number of conditions, among them pregnancy, high blood pressure, and cirrhosis of the liver.

Erythema Marginatum

Erythema marginatum is most commonly located on the extensor surfaces of the limbs—that is, the outer arm and inner leg. In most cases, it produces nodules under the skin and consequent tough projections on the epidermis of the trunks of said limbs. Otherwise, this condition is all but asymptomatic.

Erythema Toxicum

Erythema toxicum, also known as erythema toxicum neonatum, occurs in half of newborn infants, sometimes from the moment of birth. The bumps and papules that come attached with toxic erythema are of an unknown origin, but are painless, harmless and clear up naturally, this requiring no treatment.

Erythema Infectiosum

As the name implies, erythema infectiosum (also fifth disease and slapped-cheek disease) implicates human parvovirus B19, a highly infectious strain of erythrovirus. It is marked by prominent red cheeks, a lacy rash across the body, aches and fever. There is no vaccine, and little treatment beyond pain relievers and rest.

Erythema Ab Igne

Suggested by the Latin root, erythema ab igne is a direct result of overheating, or rather, overexposure to infrared radiation and consummate thermal energy. Sources include electronic devices, heaters, and hot water bottles. Recovery is contingent on the removal of the source from the person or vice-versa. 

Erythema Annulare Centrifugum

Also suggested by the use of Latin, erythema annulare centrifugum is a red ("erythema"), itchy lesioning of the skin in ring-like ("annulare") fashion from the center ("centrifugum") outward. This condition is seen in patients who have been diagnosed with autoimmune disease and infection, in particular.