The integumentary system includes the skin -- the largest organ of the body -- along with the hair, nails, and sweat and oil glands. **Like an overcoat, the skin functions as a tough barrier that protects you from injury, harmful substances and infection.

** However, exposure to the environment leaves the skin susceptible to many diseases. These include bacterial, viral and fungal infections, as well as skin growths 1.

Viral Infections

Many skin conditions are caused by viral infections 1. Warts develop due to an infection with one of the more than 100 types of the human papillomavirus, or HPV. Warts most frequently occur on the hands, feet, face and genitals. Certain strains of HPV that cause genital warts can lead to cervical cancer in women. Molluscum contagiosum is another common viral skin disease. This poxvirus infection causes clusters of small, pink, painless bumps on the skin. The herpes simplex virus causes cold sores and genital herpes.

Fungal Infections

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The skin, hair and nails contain a fibrous protein called keratin.

Several types of fungi can infect the skin and nails, and feed off this protein. Fungal infections of the nails -- especially the toenails -- are very common. Ringworm describes a fungal infection of the skin. Athlete's foot and jock itch are common names for ringworm of the feet and groin. The scalp is also a frequent site for ringworm.

Yeast is a form of fungi that likes to live in warm, moist places. So certain areas of the body are ideal environments for yeast infections, such as the groin and armpits.

Bacterial Infections

Bacteria normally live on the skin but sometimes cause infections 9. Infection of the hair follicles, called folliculitis, causes painful, pimple-like bumps filled with pus. Skin abscesses, or boils, are larger pus-filled pockets of infection that extend deeper into the skin. They are almost always caused by a bacterial infection.

Impetigo is a superficial, bacterial skin infection that results in small pus-filled bumps and crusty scabs in the affected areas. Sometimes a bacterial infection can spread into the deep skin layers, a condition called cellulitis. The infected area is red, warm, swollen and painful.

Skin Growths

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Skin growths are very common, and most are noncancerous. Nearly everyone has moles, and most older adults have skin tags. Seborrheic keratoses -- scaly, brown, flat growths -- are also common in older adults and are not cancerous. Actinic keratosis is a thick, crusty skin patch that develops from too much sun exposure and is potentially precancerous.

Skin cancer is the most common cancer in the United States. Basal cell carcinoma is the most common, followed by squamous cell carcinoma. Melanoma is the least common but most dangerous type of skin cancer.

Familial and Other Skin Diseases

Several noninfectious skin conditions run in families, but they can occur without a family history of the disease. Dermatitis, also known as eczema, is a common condition that produces a red, itchy rash. It is often triggered by exposure to something that irritates the skin.

Psoriasis also causes red, itchy, scaly skin patches that most often develop on the elbows, knees and scalp. Some people are born with little or no pigmentation, called albinism, or may lose pigment in patches with a condition called vitiligo.