This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
There are three varieties of _C. botulinum_; 2 of these varieties (A, C)
live and grow in soil under anaerobic (without oxygen) conditions, while 1
variety (E) can be found in fresh and saltwater, also under anaerobic
Under aerobic (oxygen) conditions, all varieties of _C. botulinum_ encyst,
producing a spore. Under normal *aerobic* conditions, both oxygen and your
immune system take care of the few dormant spores that you meet in everyday
life. NOTE: This is the dormant spore, *not* the bacterium. The bacterium
is what you could find in a badly processed can. However, while the
encysted, dormant form does *not* produce the toxin (only the bacterium does),
the _C. botulinum_ spore is much more resistant to extreme conditions than
the bacterium, making it harder to kill.
Deadly problems can occur in situations where you attempt to preserve food by
creating an *anerobic* state; namely, when you create a vacuum seal using
heat and a 2-piece lid, sometimes when you preserve food in oil, or when you
smoke meat. In each of those situations, the _C. botulinum_ spores can
develop ("hatch" is a good way of thinking of it) into the bacterium, which
then produce the toxin in your canned goods, oil, or on your smoked meat.
For this reason, _C. botulinum_ spores in canned/smoked food must be killed
or must be kept dormant. You, as a food preserver, using good common sense
and a bag of tricks can accomplish this.