Washing fruits and vegetables does reduce the risk of food poisoning. However, washing alone may not be enough.
Studies show that some disease-causing microbes can evade even chemical sanitizers. These bacteria can make their way inside the leaves of lettuce, spinach and other vegetables and fruit, where surface treatments cannot reach them. Microbes can also organize themselves into tightly knit packs called biofilms to protect themselves from harm.
Biofilms can harbor multiple versions of infectious, disease-causing bacteria, such as Salmonella and E. coli.
Unfortunately, the researchers conducting the linked study recommend irradiation as a way to solve the problem. Like chemical sanitizers, irradiation is soluton that is potentially worse than the original problem. Getting fresh food from a trusted local source, and washing it with water, is most likely healthier than trying to eliminate every last microbe with dangerous measures.
* Science Daily April 16, 2008