The auxiliary verb do is used in a lot of ways.

We use do to make questions with ordinary verbs, but not with auxiliary verbs. Compare:
Do you like I Can you /

We use do to make negative sentences with ordinary verbs, but not with auxiliary verbs. Compare:
I don't like football.
I can't play football.

We use do instead of repeating a complete verb or clause.
She doesn't like dancing, but I do (= ... but I like dancing.)
Ann thinks there's something wrong with Bill, and so do I.
You play bridge, don't you?

We use do in an affirmative clause for emphasis.
Do sit down. She thinks I don't iove her. but I do love her.

We can use the auxiliary verb do together with the ordinary verb do — so that we have do twice in the same verb phrase.
What do you do in the evenings?
'My name is Robinson.' 'How do you do?

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The Parasite and the Protector

An immune cell tangles with a protozoan parasite in a life or death struggle. The ribbon like parasite isTrypanosoma brucei, a microscopic menace thatcauses African sleeping sickness. The parasite is transmitted by the bite of the tsetse fly. New research, published June 14, 2012 online by the journal Science, finds that once in the body, this parasite is well adapted to give the immune system the slip. By releasing certain messenger chemicals, the parasite can shut down the anti trypanosome proteins in immune cells.