A microvirus was a genetically engineered microorganism designed to attack cells with a certain DNA sequence. These viruses attached to parasympathetic nerves, blocking the function of the enzyme cholinesterase and with it autonomic nerve impulses.
In the mid-23rd century, Yuta of the Acamarian clan Tralesta used a microvirus to murder all but one of the members of the clan Lornak, in revenge for almost exterminating her clan. Victims of this microvirus included Penthor-Mul and Volnoth. (TNG: "The Vengeance Factor")