It seems that the bite of a werewolf transmits the "condition", nobody is immune to it, and it cannot be cured.
Werewolves go on a rampage every month.
This means that during their lifetime they can transmit their condition to more people on average, than how many children normal humans can have.
How aren't there a lot more werewolves then?
I asked specifically for Harry Potter, but I'm open to answers from other fictional universes if their werewolves are very similar.