Skin is the first line of defense and encounters a variety of microbes. Some pathogenic strains are often the cause of a broad range of infections of the skin and other body systems. These conditions can affect people of all ages and may have different causes, including genetic factors, infections, autoimmune reactions, environmental factors, and lifestyle choices.

Gram-positive Staphylococcus spp. and Streptococcus spp. are responsible for many of the most common skin infections. However, many skin conditions are not strictly associated with a single pathogen. Opportunistic pathogens of many types may infect skin wounds, and individual cases with identical symptoms may result from different pathogens or combinations of pathogens.

Bacteria like S. aureus are contagious and spread quickly through skin-to-skin contact. S. aureus is often associated with pyoderma, skin infections that are purulent. Pus formation occurs because many strains of S. aureus produce leukocidins, which kill white blood cells. These purulent skin infections may initially manifest as folliculitis but can lead to furuncles or deeper abscesses called carbuncles.

Impetigo, another bacterial infection, causes the formation of vesicles, pustules, and possibly bullae, often around the nose and mouth. Bullae are large, fluid-filled blisters that measure at least 5 mm in diameter. Impetigo can be diagnosed as either nonbullous or bullous. In nonbullous impetigo, cysts and pustules rupture and become encrusted sores.

Viral skin infections by human papillomavirus (HPV) are transmitted by direct contact leading to Papillomas (warts). There are many types of HPV, leading to various presentations, such as common warts, plantar warts, flat warts, and filiform warts.

Fungal infections, also called mycoses, can be divided into classes based on invasiveness. Mycoses that cause superficial infections of the epidermis, hair, and nails are called cutaneous mycoses. Mycoses that penetrate the epidermis and the dermis to infect deeper tissues are called subcutaneous mycoses. Mycoses that spread throughout the body are called systemic mycoses. A group of cutaneous mycoses called tineas is caused by dermatophytes and fungal molds that require keratin, a protein found in skin, hair, and nails, for growth. There are three genera of dermatophytes, all of which can cause cutaneous mycoses: Trichophyton, Epidermophyton, and Microsporum.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 21.2 bacterial infections of the skin and eyes; Section 21.3 viral infections of the skin and eyes; Section 21.4 mycoses of the skin