You can catch pubic lice - or 'crabs' as they're otherwise known - through close body contact, such as during sex, because condoms (whilst offering effective protection against most sexually transmitted infections) cannot prevent infestations of lice. Pubic lice are tiny grey-brown insects, about 2mm long, that live by sucking blood from your skin. Nobody wants little suckers like that.
Pubic lice like living in hairy areas of the body. Because they prefer thick, coarse hair, they are commonly found in the pubic area but can also be found in hair around the anus, armpits, beard, chest and occasionally even eyelashes or eyebrows.
How to Avoid
The best preventative measure is to refrain from having sex with people who might have crabs (which, in theory, could be anyone). Of course this isn't realistic, so being careful and vigilant, and seeking treatment if you think you've got crabs, is the best you can do.
If you now feel like hitting the shower and scrubbing yourself vigorously, the bad news is it won't wash the lice away. If you suspect you've got a lice circus in your underwear then you need to consult your GP, who may prescribe an insecticidal lotion or cream.