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Every 24 hours after becoming a vampire, you will progress to the next "Stage" of vampirism. There are four possible stages, with increasing bonuses and negative effects. Feeding will always return you to Stage One.  Each stage will increase your resistance to Frost, but will increase your susceptibility to Fire. The damage you take from sunlight increases each stage. Each stage grants you access to more vampiric spells and increases your vampiric strength. NPCs will become more hostile towards you as the stages increases, attacking you on sight at Stage Four. Sunlight will hurt you, especially if you are in the later stages of vampirism. Night-time travel will also help keep you out of sight of most NPCs. Make good use of your vampiric abilities to hide yourself from sight. If you want to avoid most of the negative effects of vampirism, you will need to feed regularly. If you are playing standard Skyrim, you can feed on sleeping people by approaching them and pressing the interaction button as if you were Pickpocketing. Another option will appear, allowing you to Feed.  If you're playing Dawnguard, you can feed on a conscious person if you cast the Vampire's Seduction spell first. Feeding will cause witnesses to become hostile, and will incur a 40 gold bounty if you're caught. Feeding on your spouse is one of the easier ways to manage your bloodthirst. If you've had enough of being a vampire, you can undertake a special quest to reverse the effects. First, ask any bartender for rumors and you will be told about Falion, who studies vampires. You can find Falion in Morthal.  Falion will inform you that vampirism can be reversed with a filled Black Soul Gem. You can fill a Black Soul Gem by casting Soul Trap on a humanoid opponent before defeating it. Falion will sell you an empty Black Soul Gem if you need one. Turn filled Soul Gem over to Falion and he will remove your vampirism. This can be repeated as often as you'd like, as long as you have a filled Black Soul Gem. If you can't get to the quest and you're desperate to revert your vampirism, you can use the console to cheat and remove it instantly. Open the console by pressing ~. Type player.addspell 301462a and press ↵ Enter. Then type showracemenu and change your race. Vampirism will be cured immediately.

Summary:
Balance your positive and negative effects. Travel at night. Feed to quell your bloodthirst. Cure your vampirism. Cure your vampirism with the console (PC only).