Red Dust or V-Dust was the name given to a biological weapon developed by the human Resistance for use against the Visitors in the original series.
While incarcerated aboard the Los Angeles mothership, 17-year old Robin Maxwell was impregnated by the Visitor officer Brian as part of an experiment in cross-species breeding conducted by Diana. Shortly afterwards, however, Robin was rescued by Mike Donovan with the aid of the Visitor fifth columnist Martin, and taken to the headquarters of the Resistance.
Nine months after her rescue, Robin gave birth to fraternal twins, one of whom had mainly human characteristics while the other shared the reptilian traits of the Visitors. Shortly after its birth, the reptilian infant died. While conducting an autopsy, Dr. Juliet Parrish and Dr. Robert Maxwell discovered a previously unknown strain of bacteria in the infant's body which had been responsible for its death. This bacteria would form the basis for the Red Dust, and would prove to be a pivotal weapon against the Visitors.
In its final form, the toxin, as its name suggests, was a powdery, blood-red substance. When released into the atmosphere, the Red Dust bacteria would multiply and insinuate themselves into the earth's biosystem, rendering all terrestrial life forms and resources permanently unusable to the Visitors.
It was initially thought that the bacteria was harmless to terrestrial life; however, it was soon found that in sufficiently high concentrations, Red Dust could cause sterility, genetic mutations, and even death in humans, animals, and crops. It was also discovered that the bacteria needed a dormant (i.e., cold weather) period to regenerate and flourish; in continually warm climates, the bacteria would quickly begin to lose effectiveness after its initial release.
When ingested by the Visitors, the bacteria would swiftly begin multiplying and excreting waste products which interfered with lung function, leading to death from respiratory failure within minutes. To protect the Visitor fifth columnists who were aiding the Resistance, an antitoxin was developed which could counter the effects of Red Dust exposure for brief periods.