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Botulinum Toxin:

A product of the bacterium Clostridium Botulinum, this toxin is the single most toxic substance (on a weight basis) in the world.

The dose that is lethal to 50% of the population exposed (LD50) is just 1 ng/kg of body weight. In other words, just 60 ng ( or 0.00006 grams) would have a 50% chance of killing a 60 kg man.

Looking at it from yet another angle, 1 kg ( yes, One) of this stuff if exposed properly to 16 MILLION people will kill half (or 8 MILLION) of them!

*See the Calculation below (and let me know if I am wrong.. I don't think I am though..)

                        0.00006 grams --------- 50% chance of killing a 60kg man.
                        thus, 1 gram ------------ 50% chance of killing 1/0.00006 men.
                        thus, 1 kilogram ------- 50% chance of killing  (1/0.00006) X 1000  men.

 (1/0.0006) X 1000 = 16,666,666.6666666666666666666666667 or approx 16 (actually 17) million.

All the Botulinum toxins (there are 8 varieties with varying toxicities, the figures given above are for the subtype that is most toxic) are neurotoxins. They derange the physiology of the Neuromuscular junctions by acting on the presynaptic terminals, ultimately causing gradually increasing paralysis in all parts of the body.

Images: 
Please click to view larger versions.

1) The bacterium that produces botox, Clostridium botulinum.
2) Ptosis (drooping of the eyelid due to paralysis of the lid muscles)
3) A patient placed on an artificial respirator. ( <100 of these in KEM)

The eyes are affected initially, with the ocular movements becoming retarded. This progresses to ptosis. On around the fourth day, speech becomes indistinct  and gait uncertain as patients complain of extreme weakness. The disease may go on to involve the respiratory system, rendering the patient incapable of breathing by himself, as a result of which he may need to be placed on a respirator. Because the disease is generally self-limiting, the case fatality rate is very low (~10%) IF top-notch medical care including artificial respirators is available.
Recovery generally occurs in 10-14 days.

Manufacturing Ease:

As Biological agents go, Botulinum is extremely easy to produce in weapons grade quantities. Even a small country like Iraq had produced 19,000 liters of concentrated botulinum toxin by the time the gulf war was over.

However, please note that 19,000 liters here does NOT correspond to 19,000 kg of the stuff. I don't know the exact concentration of the Iraqi liquid, but it is surely not 1kg/lit.

Ease of Delivery:

Quite easy to deliver actually. Since it is not a living substance, not too many precautions need to be taken while transporting botulinum.

However, botulinum does get inactivated by sunlight (within 1-3 hours)

Also, botulinum is inactivated by contact with chlorine (84% inactivated by 20 minutes at 0.4% mg/L FAC). Hence, there is no reason to be afraid of someone spiking your water supply with some botulinum. At least not until the Bombay Muncipal Corporation's water guys are doing their job well.

Infectivity:

Botox is a non-living substance :) . The fatal dose has already been mentioned.

Mortaility:

Didn't I just mention that the fatality rate for Boulinum with proper care was around 10%? Why the high score here then?

Patients having respiratory paralysis need to be kept on artificial respirators, without which they would DEFINITELY die. I haven't counted the exact number of respirators in KEM, but have seen quite a few fights between the registrars for getting respirators allotted to their patients. All in all, I don't think the total number of respirators in KEM exceeds 100.

If a moderate quantity (say 100 gms) of Botulinum toxin were to be distributed perfectly over Bombay, around 8.3 million people would be at risk of dying. Granted, not all of them would develop respiratory paralysis, but would there be enough respirators for those who did? Probably not.
For a similar dilemma, perhaps explained in a more convincing manner, read "The Bioweaponeers" from the "Reading Material" section of the article. 

Interesting Facts About Botox :

1) One more thing about the toxicity of Botox. It is15,000 times more toxic by weight than VX (the dreaded chemical warfare agent) and 100,000 times more toxic than Sarin (the agent that AumShirinkyo released in the Tokyo subway).

2) Before Aum Shirinkyo (the fanatical Japanese cult) released Sarin into the subways of Tokyo, they had tried may times, unsuccessfully, to release botox.

3) It is believed by some that Reinhard Heydrich, head of the Gestapo and Security Service in Germany during World War II and a person who was called "The Butcher of Prague" by his detractors because of his central role in the holocaust was assassinated by using botox.

Heydrich was attacked by Czech nationalists in Prague, with a grenade that had been kept below his car and which when it exploded drove fragments through his seat and into his lung, diaphragm and spleen.
He however survived and recovered excellently, but his condition suddenly worsened on the 7th post operative day. The next day, he was dead.

Image : "The butcher of Prague" . Please click on the image to view a larger version.

The German pathologists noticed no life-threatening pathologies during the autopsy. The autopsy report stated that : "...death occurred as a consequence of ... bacteria and possibly by poisons carried ... by the bomb splinters...."

Click here to read more about the incident. Includes some information that apparently supports the theory.
(Note : I have not written this article myself. It is from the internet. However, this is not the work of a single over imaginative mind. There are many references to the incident on the net.)

Personally, I think it's all baloney.