A curse, the evil eye, or a hex. Sometimes talismans or poppets are used to bring bad luck, however, an ethical practitioner rarely makes such a thing.

Monkey bring bad luck not good luck.

The character who brings bad luck to Jonah Cain in "Atlas Shrugged" is Hugh Akston. Hugh is a philosopher and educator who becomes involved with the looters in the novel, which ultimately leads to negative consequences for Jonah Cain and others.

a snake cant bring you bad luck unless it is a a adder that is black then it is a year of bad luck

They were once thought of to bring bad luck or to have the 'eye' of god(s) watching you.