A parasite lives off another being without contributing anything to that other being.

Malicious means having bad intentions.  If you treat someone maliciously you are trying to hurt their feelings.

Vegetation is plants.

Adverse means negative -- something that is against the way you want things.

Deteriorates means falls apart or gets worse.

A discrepancy is the difference between how something is and how it should be.  If you're supposed to have $50 and you only have $45 there's a discrepancy of $5.

A negative connotation is a negative feel to a word that does not really show up in its dictionary definition.  it's not a comment, it is a meaning to a word.  Like the word "black" has negative connotations in our society even though it is just a color.