Although the volunteers will excrete the vaccine in their faeces, it will be destroyed by the sewage treatment system.

BBC: Go-ahead for GM diarrhoea vaccine tests

Other times the attacks are more personal:In 2000 a discontented consultant, rejected for a job at a water treatment plant in Australia, remotely hacked into a sewage treatment system and released 264, 000 gallons of raw sewage into rivers and parks.

FORBES: The Next Threat