Why is a Werewolf not immune to Sanguinare Vampiris?

After achieving Level 30 (+/-) in Skyrim, I installed the Dawnguard DLC and surprisingly in one of the Vampires' invasions (in Markarth, if I'm not wrong) I contracted Sanguinare Vampiris from a Volkihar Vampire, which shouldn't be possible as I am a Werewolf.

I would like to know why or how this happened.
Isn't a werewolf immune to diseases because of the beast blood?

Solution 1:

Make sure you keep the latest savegame before that contraction!

The uesp wiki states the following:

You can also cure your vampirism by becoming a werewolf through the Companions faction. However, once you contract lycanthropy, you gain a 100% resistance to disease and cannot be re-infected with Sanguinare Vampiris by normal means while a werewolf.

and

The first time you transform using Beast Form, you will immediately be cured of all diseases, including any stage of vampirism. This means you cannot be a vampire and a werewolf at the same time. The Beast Form power also automatically removes all of your equipment, and all summoned creatures (including dead thralls) will immediately die.

but

Accepting Lord Harkon's gift of vampirism during the Awakening quest or asking Serana to make you a vampire (either during the Chasing Echoes quest or after the end of the Dawnguard questline) will also cure you of lycanthropy.

So there are about three possibilities:

• You will be cured of vampirism when turning into beast form, without ever becoming an actual vampire
• Volkihar vampirism is indeed stronger than lycanthropy and you are actually no longer a werewolf
• By a bug similar to

If a non-werewolf contracts Sanguinare Vampiris but has not become a vampire yet, becoming a werewolf will not cure vampirism. This will progress to the player being both vampire and werewolf at the same time. Since the werewolf feeding ability overrides that of the vampire, the player cannot feed as a vampire and there will be no way to prevent progressing to stage four vampirism.

you have successfully managed to become a Wermpire / Vampilf...