Checking buildings for indoor air quality or other contaminants which may affect occupant health should not omit inspecting and taking site history for evidence of sewage or septic system backups into the structure or basement or crawl space areas below the structure. Gray water and black water (sewage) can cause a wide range of fungal, bacteriological, viral, and parasitic hazards in buildings. Both a one-time event and recurrent sewage leaks into a building would be a concern, particularly if prompt and competent cleaning were not performed. If recurrent sewage contamination has occurred more extensive building cleaning and treatment are likely to be required. One reason that experts recommend very prompt treatment is the wish to avoid transmission of bacterial contamination to other building areas. Examples of sewage bacteria and virus transmission might be by movement of people from contaminated areas to other building areas (tracking contaminated soil), and air movement of aerosolized particles or contaminated dust through the building by natural convection, heating and air conditioning equipment, or other sources of air and dust movement.