How do you become a werewolf in elder scrolls 3 morrowind for PC

Fight a werewolf, there is a chance it will inflict Sanies Lupus on you if it injures you. However, if you have reached a certain point in the main quest, you will be immune to disease and can't catch it.

At one point in the Bloodmoon main quests though, you will automatically be infected by it, and at the end, there is the Ring of Hircine which can turn you into a werewolf.