The nail disease paronychia (commonly misidentified as a synonym for felon or whitlow, the latter two of which refer to a deep space infection of the distal end of a digit) is an often tender bacterial or fungal hand infection where the nail and skin meet at the side or the base of a finger or toenail. It can start suddenly (acute paronychia) or gradually (chronic paronychia).
Dr. Rob Hicks writes on the BBC website:
The cuticle acts as a protective seal but if it's damaged in any way bacteria can enter the skin and cause infection. These infections can be extremely painful as the skin becomes inflamed, hot, red and throbs continually.
Pus is usually present, along with gradual thickening and browning discoloration of the nail plate.
If a large amount of pus has collected, then it may be necessary to make a small cut in the skin (sometimes under local anaesthetic) to release it.
Acute paronychia is usually caused by bacteria. This is often treated with antibiotics, sometimes as a cream, other times orally. Chronic paronychia is most often caused by a yeast infection of the soft tissues around the nail but can also be traced to a bacterial infection. If the infection goes on and on then a fungal infection is often the cause and this needs anti-fungal cream or paint to treat it.
Hicks writes further about the causes:
Whitlows are common, especially for people who have to repeatedly wash their hands. Excess water weakens the seal, while soaps and detergents remove the protective skin oils leaving the skin dry and more liable to split.
Most often, trauma to the cuticle allows infection in. Biting or picking at the cuticle, damage through work and overenthusiastic manicuring are the usual culprits.
Individuals who work with their hands in water, such as health care workers and food processors, are quite prone to the fungal type of infection.
Herpes whitlows are frequently found among dentists and dental hygienists. Prosector's paronychia is a primary inoculation of tuberculosis of the skin and nails, named after its association with prosectors, who prepare specimens for dissection. Paronychia around the entire nail is sometimes referred to as runaround paronychia.
Painful paronychia in association with a scaly, erathematous, keratotic rash (papules and plaques) of the ears, nose, fingers, and toes, may be indicative of acrokeratosis paraneoplastica, which is associated with squameous cell carcinoma of the larynx.