A Louse

1. A small parasitic wingless insect of the order Psocodea.
2. (colloquial, dated, not usually used in plural form) A contemptible person; one who is deceitful or causes harm.

When used as a term of abuse, the plural is typically louses, though lice is also possible.

(insect): (North America) cootie

(contemptible person): maggot, worm