Itchy skin may be caused by a variety of different skin conditions and disorders, including psoriasis, eczema or dermatitis, hives, chickenpox and infestations with lice or scabies, notes Mayo Clinic. Itching from these conditions is usually generalized to certain areas and occurs along with skin irritations, redness, blisters and bumps.
Itching and rash also accompany reactions to certain drugs, notes Mayo Clinic. For example, narcotic pain remedies, antifungal drugs and antibiotics often cause widespread rashes and skin itching.
Rashes occur when the skin is irritated, notes WebMD. Because the skin is a barrier between the outside environment and the body, a rash indicates a change to the affected skin. Contact dermatitis is a common cause of rashes, and it occurs when the skin comes into contact with an allergen. The rash is the body's response to the allergen. This is often a poisonous plant, such as poison oak, ivy or sumac or contact with lotions, cosmetics, detergents and soaps.
Viral infections, including those caused by herpes zoster, often cause rashes and itching, reports WebMD. Candida albicans, impetigo and sexually transmitted diseases are other causes of rashes. Rashes may also indicate something more sinister, such as some types of cancer or kidney or liver diseases.