As a previously-unknown bacteria and therefore one not initially vaccinated against, E. coli mutagena dealt a major blow to the Terran forces, resulting in a temporary retreat. Attempts to create a stable and mass-reproducible inoculation have been unsuccessful to date.
The bacterial weapon was first discovered by known Resistance leader Doctor Juliet Parrish a year after the initial Terran Invasion. The organism was a variant of E. coli found in the digestive tracts of Robin Maxwell’s hybrid children, unfortunately fatal to the more reptilian twin. The Resistance manufactured the bacteria in mass quantities, encapsulated like yeast, for distribution. Re-hydration would reactivate the organism for inoculation of food and water chains. As a result, the Earth was (temporarily) fortified against Sirians.
The red dust is fatal to Sirians, not by infection, but rather by allergic reaction. Flagella of the bacteria possess "O-antigens" which trigger an immediate allergic reaction upon contact with skin, eyes, or the respiratory tract of Sirians. Possibly, the massive vaccinations to Terran fleet personnel have hypersensitized them to allergens. Symptoms of the common human cold and respiratory ailments were reportedly problematic among many fleet personnel.
Further research at Science Frontiers revealed that the exotoxins of the red dust, in high concentrations, are also toxic to humans. The red dust caused a deadly side-effect in terrestrial agriculture, producing blight in food crops such as corn and wheat. Other side effects include mutations and stillbirths among animals, and marked increase in respiratory ailments among humans living in areas with high concentrations of red dust deployment.
In addition, periodic seasons of cold temperatures are necessary for the mutant strain of the bacteria to regenerate and remain effective. Consequently, the warmer regions of Earth have lost the protection of the red dust. Because the side effects prevent the humans from constantly redeploying the bacteria, the path for re-invasion has been opened.