Crabs are small insects which live in coarse body hair. They are more common than generally realised - and certainly do not imply lack of hygiene. They're passed on by intimate body contact - direct contact is probably required. Although they can be transmitted by sharing bedding or clothing, this is probably very much rarer.
Symptoms you might notice include itching around the pubic hair area, minute specks of blood on the surface of the skin, the tiny insects themselves or their eggs (small white dots fixed to the hair).
How do you treat them?
Crabs are very easily treated with a lotion which should be applied to all coarse hair bearing areas below the neck. It is left on for a few hours and then washed off. There is no need to shave the affected areas. To prevent re-infection, wash all clothing and bed linen which has been in contact with the area. If you're in a regular partnership, your man needs treatment too.