A paronychia is an infection which develops next to the fingernail. It may be a mild infection, but often is associated with a small abscess - a collection of pus beneath the skin. A paronychial infection may develop suddenly due to a bacterial infection, or may persist for months or more due to a fungal infection.
Paronychial infections are common because of the way our fingernails are made.
The skin which overlaps the nail is usually stuck down to the outer surface of the fingernail. The cuticle is the edge of where the skin is usually attached. However, there is a little area between the covered portion of the nail and the skin which covers it - and this area may provide a shelter space space for bacteria to grow. We all have bacteria on our hands, even after we wash, and bacteria or fungus may hide and grow in small crevices next to the fingernail. Occasional paronychial infections are probably impossible to prevent.