What Am I Riddles

Question: I am a big threat to humanity,
but you wouldn't get rid of me even if you could.

What am I?

Question: I go into the water red and come out black.

What am I?

Question: I give advice to others,
yet I know nothing myself.
I am a hitchhiker destined to stay still.

Question: There are two sisters: one gives birth to the other and she, in turn, gives birth to the first. Who are the two sisters?

Question: What has four legs and can't walk?