During the week of June 4, 2001, Houston and surrounding areas experienced what has been called "the Flood of the Century" due to constant inundating rains deposited by Tropical Storm Allison.

Post-flood surveillance of water-borne disease thereafter became the primary focus of all efforts by the Bureau for the remainder of the month. Beginning on the evening of June 9, the Bureau began telephone surveillance of emergency rooms for reported cases of gastroenteritis. By June 10, the surveillance had grown to cover 13 hospitals, and over the next two weeks it had broadened to include six American Red Cross shelters, three emergency clinics, one emergency hospital, and an HDHHS multi-service center. The Bureau specifically conducted surveillance of reported cases of gastrointestinal illness, respiratory illness (secondary to mold, and organic dust inhalation) and attempted to assess the demand/need for tetanus vaccination throughout the city. Active surveillance of hospitals and shelters was suspended on June 25, after a review of data collected demonstrated no unusual increases in conditions being monitored.

In addition to active assessments of hospital emergency centers and shelters, the Bureau coordinated a Rapid Needs Assessment following tropical storm "Allison". On Saturday, June 16, representatives of HDHHS, Texas Department of Health (Headquarters and Regional office 6/5 South), the Centers for Disease Control and Prevention and UT School of Public Health conducted a randomized survey of 60 census tracts in Houston/Harris County affected by tropical storm Allison. Over 70 volunteers logged more than 600 hours for the needs assessment, not including the many hours spent planning and organizing this massive assessment effort since June 13th. A total of 420 interviews were conducted utilizing a standardized questionnaire (60 census tracts x seven interviews/census tract). The Bureau in conjunction with UT School of Public Health conducted a preliminary analysis of the data, which was distributed to the Office of the Mayor, members of City Council, the Office of Emergency Management, the American Red Cross, FEMA, CDC’s Emergency Response Team and the Texas Department of Health. The findings will be published in the MMWR.

The information collected through the Rapid Needs Assessment survey has been used to assess the damage, to direct assistance efforts and will be used for planning for future disaster responses in Houston, as well as to guide future rapid assessment efforts in the area.