Virus: an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host. Amoeba: a single-celled animal that catches food and moves about by extending fingerlike projections of protoplasm. Bacteria: a member of a large group of unicellular microorganisms which have cell walls but lack organelles and an organized nucleus, including some which can cause disease. Toxin: a poison of plant or animal origin, especially one produced by or derived from microorganisms and acting as an antigen in the body.