It occurs upon contact of a (human) body part with any source of electricity that causes a sufficient current through the skin, muscles, or hair.
An electrical injury has many consequences to a body as the electrical currents can travel through the nervous system and burn out tissue in patches along the way.
Static electricity happens more often during the colder seasons because the air is drier, and it's easier to build up electrons on the skin's surface. In warmer weather, the moisture in the air helps electrons move off of you more quickly so you don't get such a big static charge.
