Bacteria cause most types of finger infections. The exception to this is the herpetic whitlow, which is caused by a virus. How the infection starts and is found in a particular location is what makes each specific type of infection unique. Usually, some form of trauma to the finger or hand is the initial event. This may be a cut, animal bite, or puncture wound.

• Paronychia: The offending bacteria are usually staphylococcal and streptococcal organisms. Rarely, a fungus causes this infection, which usually begins as a hangnail. Often a person will attempt to bite off the piece of nail that is at the corner of the nail, leading to local infection. This infection can also be caused by aggressively shoving back or trimming cuticles. This results in an open wound that allows the bacteria on the skin and the bacteria in the mouth to infect the wound. The infection can then spread to the surrounding tissue next to the nail and cuticle.
• Felon: This bacterial infection of the finger pad, caused by the same organisms that cause paronychia, is usually the result of a puncture wound. The wound allows the introduction of bacteria deep into the fingertip pad. Because the fingertip has multiple compartments, the infection is contained in this area.
• Herpetic whitlow: The offending viral organism is the herpessimplex virus type I or II. This is the same virus that causes oral or genital herpes infections. People in certain occupations are more at risk for this infection. These include dentists, hygienists, physicians, nurses, or any other person who may have contact with saliva or body fluids that contain the virus. People with oral or genital herpes may also infect their own fingers.
• Cellulitis: The most common causes of this bacterial infection are staphylococcal and streptococcal organisms. This infection is usually the result of an open wound that allows the bacteria to infect the local skin and tissue. The infection can also spread to the hand and fingers by blood flow carrying the organisms.
• Infectious flexor tenosynovitis: This bacterial infection is usually the result of penetrating trauma that introduces bacteria into the deep structures and tendon sheaths, which allows the spread along the tendon and associated sheath.
• Deep space infection: This bacterial infection is usually the result of a puncture wound or deep cut that introduces the bacteria to the deep tissue. The collar button abscess is associated with the web space between the fingers. The deep structures of the hand create many potential compartments for this infection to invade. 

There are several risk factors for developing finger infections, including the following:

• Puncture wounds
• Deep cuts
• Any open wound
• Penetrating trauma
• Picking hangnails
• Nail biting
• Aggressive manicuring or cuticle trimming

Some individuals may be at higher risk for finger infections, including

• people with diabetes;
• people with compromised immune systems;
• people in certain occupations, such as dentists, hygienists, physicians, and nurses; and
• people in occupations involving extended exposure of the hands to water (such as dishwashers).

Each of the main finger infections has specific signs and symptoms that make identification unique and may cause confusion if not properly evaluated.

• Paronychia: Symptoms and signs of paronychia include redness and swelling of the area next to the fingernail. A visible collection of pus may be seen under the skin and nail. This fluid may be leaking out of the wound. The area will be tender and painful to the touch. The drainage from the area is usually a cloudy white-yellow color.
• Felon: Symptoms of a felon include a swollen and painful fingertip. The swelling usually develops over several days and is located in the pad area of the fingertip. The area will have a throbbing pain and be painful to the touch. The area is usually red, and a visible collection of pus may be seen under the skin. The swollen area may have a portion that feels soft as if it contains fluid. As the swelling continues, the area may become tense or hard to the touch.
• Herpetic whitlow: Symptoms of herpetic whitlow include redness and tenderness of the fingertip area. A burning or itching sensation may be present in the area. There may be mild swelling but not as extensive as in the felon. There may be a single or many open wounds in the area affected. These open wounds often occur in clusters after the formation of a small blister-like lesion. The fluid in these lesions is usually clear in appearance but may be slightly cloudy. The affected individual may also have a low-grade fever and have swollen and tender lymph nodes in the area.
• Cellulitis: Symptoms of cellulitis include skin area redness and warmth to the touch. The area may be slightly swollen and tender. This is usually a superficial infection so the deep structures should not be involved. The motion of the fingers and hand should not be difficult or painful. If painful or difficult, this may indicate a deep space infection of some type.
• Infectious flexor tenosynovitis: Four major signs often are found with infectious flexor tenosynovitis.
  • First is tenderness over the flexor or palm side of the finger. This pain is found over the tendons in the finger.
  • Second is uniform swelling of the finger.
  • Third is pain on extending or straightening of the finger.
  • Fourth, the finger will be held in a slightly flexed or partially bent position.

These signs are called Kanavel cardinal signs. All four signs may not be present at first or all at once.

• Deep space infections: The deep space infection that arises in the web space of the fingers is also called a collar button abscess. Symptoms and signs of deep space infections include pain and swelling in the space between the fingers. The area may also be red and warm to the touch. As the abscess becomes larger, the fingers will be slightly spread apart by the increasing pressure. The central area may have a soft spot that represents a collection of pus under the skin.