Generally speaking, people develop West Nile Virus when bitten by an infected mosquito.  When the mosquito inserts its proboscis (see images below) through your skin a small amount of saliva is injected.  Compounds in the saliva prevent your blood from clotting, thus making it easier for the mosquito to take your blood.  Unfortunately, this small amount of saliva may contain tiny viruses, like West Nile Virus.  If enough virus makes it from the mosquito into your body, you may become infected with the virus.