The term “lesion” is very broad; it can refer to any localized area of damaged tissue or tissue that simply looks different from the surrounding tissue whether it is internal or external or caused by physical trauma, infectious agent, or inflammatory response. Red lesions of the skin are common symptoms for many conditions such as psoriasis, syphilis, and allergic reactions. The common causes of red lesions generally depend on associated symptoms like fever, scaling on the skin, or the shape of the lesion.
Red lesions that form rings are called annular lesions and have red borders with a lighter center. They can indicate a fungal infection such as tinea corporis or ringworm, granuloma annulare which form bumpy rings with no known cause, or erythema migrans which is the rash that forms in a small percentage of people in the early stages of Lyme disease. Pityriasis rosea is another condition that can cause annular red lesions in the beginning stages and is suspected to be caused by a virus. Hives — known medically as Urticaria — are an inflammatory immune system response that can form annular rings.
Another diagnostic feature of red lesions is whether they contain a small, raised area known as a papule, a larger, raised area called a plaque, or a discolored but flat area termed a macule. Warts, acne, insect bites, or stings, and some skin cancers can all cause papules. A plaque can be caused by allergic reactions, psoriasis, seborrheic dermatitis, or eczema. Flat birth marks, freckles, and even tattoos are considered to be macules along with those conditions such as measles, rubella, rickets that are caused by illness. Rosacea usually includes both macules and pustules, which is another lesion formation of small red bumps with fluid and pus inside them.
In some cases, the diagnosis of red lesions is helped by the presence or absence of scaling. Psoriasis lesions can be red, but are often covered with thick white to silver patches of scales. Seborrheic dermatitis, fungal infections, and eczema are all characterized by the likelihood of scaling. In other cases, the location of the red lesions may suggest a cause. The secondary stage of syphilis can produce rashes anywhere on the body, but the most common location is on the palms and the bottoms of the feet. Seborrheic dermatitis will show up most in areas that tend to be oily such as the scalp, forehead, and around the nose because those areas have the highest concentrations of sebaceous glands.