Dr James Howenstine says;

(Read full report at ; http://www.newswithviews.com/Howenstine/james28.htm )

Death of many healthy intestinal bacteria damages the function of the lining
membrane of the intestine. When this membrane has been injured partially
digested proteins, toxins, undigested food particles and infectious substances
can penetrate the normally protective intestinal lining. These “foreign”
substances can initiate allergic responses to food, start infectious problems
and lead to autoimmune reactions against the “foreign substances”.
Unfortunately healthy intestinal bacteria can be killed by many commonly
experienced exposures:

Courses of antibiotics kill healthy intestinal bacteria leaving dangerous
anerobic bacteria, yeast, fungi and mycoplasma to proliferate replacing the
desirable intestinal bacteria. This may result in deterioration in immune
defenses permitting new infectious diseases to occur easily and frequently.

Chlorine used to sterilize drinking water kills healthy bacteria

Non steroidal anti-inflammatory bacteria kill healthy intestinal bacteria.
These drugs are widely used in inflammatory diseases.

Fluoride used in water for its supposed ability to prevent cavities is toxic
to the good intestinal bacteria.