All of these are tiny insects or mites which live in or on the hair or skin:
Scabies are tiny mites which burrow under the skin to lay eggs and leave droppings, which cause itching. They like creases or folds of skin and usually begin in the webs of fingers, spreading up the arms, over the trunk and legs. The mite that causes scabies is acquired primarily through close personal contact. It can be picked up from clothing, towels or linen.
Crabs are larger and live on the hair in the pubic area, under the arms or on the eyelashes. In very hairy persons, they can also live in other hairy places.
Head lice live in the hair on your head. They can be detected by looking for eggs or nits which are small white dandruff-type flakes that are attached to the hair shaft within one inch of the scalp. They are often easily seen at the back of the neck by pulling the hair up.