I wonder if it's a test? We know that the US is spending billions on "bio-defense" - maybe they're testing how a pathogen/chemical weapon would spread if released in a terrorist attack. We know that they've tested bioweapons before in US cities, so it's not unprecedented. This would also help explain why a cause was never found.
"Cities were unwittingly used as laboratories to test aerosolization and dispersal methods; Aspergillus fumigatus, B. subtilis var. globigii, and Serratia marcescens were used as simulants and released during experiments in New York City, San Francisco, and other sites. Concerns regarding potential public health hazards of simulant studies were raised after an outbreak of nosocomial S. marcescens (formerly Chromobacterium prodigiosum) urinary tract infections at Stanford University Hospital between September 1950 and February 1951, following covert experiments using S. marcescens as a simulant in San Francisco.
A series of field tests took place under the auspices of the Biological Laboratories from 1943 to the mid-1960s:
- In one such test, travelers at Washington National Airport were subjected to a harmless bacterium. Traps were placed throughout the facility to capture the bacterium as it flowed in the air. Laboratory personnel, dressed as travelers carrying brief cases, walked the corridors and without detection sprayed the bacterium into the atmosphere.
- In the New York Subway, a light bulb filled with the same harmless bacterium was dropped on the tracks. The organism spread throughout the system within 20 minutes. Traps and monitoring devices showed the amount of organism — if it were one of the predictable, dangerous organisms, could have killed thousands of persons. No one was injured or became ill as a result of the test.
- In San Francisco, a U.S. Navy ship, equipped with spray devices operated by Fort Detrick personnel, sprayed serratia marcescens, a non-pathogenic microorganism that is easily detected, while the ship plied the San Francisco Bay. It spread more than 30 miles to monitoring stations.
After the termination of the offensive biological program, the US Army Medical Research Institute of Infectious Diseases (USAMRIID) was established in order to continue the development of medical defenses for US military members against potential biological attack. USAMRIID conducts research to develop strategies, products, information, and training programs for medical defense against potential biological weapons.
http://www.globalsecurity.org/wmd/systems/bw.htm