How did clostridium botulinum get in my food?

Botulism is thankfully a rare disease because it cannot be transmitted from person-to-person and also because most people properly prepare their food. However, clostridium botulinum still persists in nature (soil) which means it is important to be aware. Botulism is caused by neurotoxins that the bacterium clostridium botulinum produces. Even a micro-liter could be lethal.

These gram positive, rod-shaped microorganisms are obligate anaerobes, so they thrive in the airtight atmosphere that home-canning of fish/vegetables provide. It is in this environment that the spores are able to release the toxins. Consumers should pressure cook food items before canning because high temperatures are not enough to denature the spores. 

Once ingested, it would take 8 to 36 hours for symptoms to show. Symptoms include: difficulty breathing and swallowing, nausea, and weakness with paralysis. The c. botulinum toxin essentially blocks the activity of acetylcholine--a neurotransmitter responsible for muscle activation--which is how botulism leads to paralysis and respiration difficulties.

The above diagram illustrates how the botulinum toxin (heavy chain) binds to the neuronal membrane of the neuromuscular junction. The light chain of the toxin cleaves proteins of the synaptic fusion complex. With the toxin introduced, the complex is not longer able to allow the synaptic vesicles (which release acetylcholine) to fuse with the membrane and the muscles are now paralyzed. 

Sources:
http://en.wikipedia.org/wiki/Botulism

http://en.wikipedia.org/wiki/Botulinum_toxin
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001624/
http://www.cidrap.umn.edu/cidrap/content/bt/botulism/biofacts/botulismfactsheet.html#_Therapy_for_Botulism